Welcome aboard. While you can potentially improvise everything, if you are truly new, I recommend planning as much as you can ahead of time. You can always abandon those plans if you need to do so during play ( like when the players decide to do something unusual). For first Ed resources, looked at pirate nations or waves of blood for pirate ideas,. The compendium and Explorer's society for syrneth ideas, and the Temple of the Rose and Cross website for adventures. But this system is very different in style, it is more open ended and needs less concrete mechanics. But you do need to know ahead of time where you are planning scenes and sequences to prepare consequences.

I'd have to agree with Salamanca - you do need to prepare ahead of time (if only in point form) what the main highlights of the adventure/campaign is going to be. You won't be able to account for everything that the PC's do, so be prepared to go off-road when necessary. I'm looking to run a homebrew campaign with much the same theme, and I can tell you that the resources that Salamanca suggested are very good starting points to work with. 

You can never be prepared enough, but do the best you can, and let the dice fall where they may.

And do not be afraid to admit you screwed up on some rules. Leave it as you used it for the moment and when you get to a break in play or between sessions, look it up or consult us here. Then explain to the players how it should work from that point on.

Tips for a session: 1- Start with conflict. Toss them into the middle of a chase, fight or other action to get the adventure off to an exciting pace. Think of it like a James Bond movie, it may not even heavily tie to the rest of the session (cooler if it does). But that opening stunt is always talked about. 2- give them opponents they can interact with. Sure, a monster is a good threat but players like a bad guy they can taunt, negotiate with and face mentally as well as physically. 3- if your opening g conflict doesn't do it, your next scene needs to set the stage for the adventure so players have am objective. Followed by some investigation into the problem, another fight (because we are swashbucklers), ending up in a tight spot, getting loose, and saving the day. You can mix the order on that, add extra steps and cheat them out of victory on occasion but that is the standard outline. 4- give your NPCs personality and quirks the players will remember. Does the Professor scream like a girl when scared? Does the villian's henchman have fat sausage shaped fingers? Does the Vodacce bad guy always speak with a terrible French accent to taunt the Montaigne hero? Players latch onto this stuff and remember it. 5- you can never prepare for what the players end up doing. They will go "left" if you plan "right". So make your plot, know where you need them to end up and spend time thinking of ways to get them back on course by the end of the session. What happens in between does notatter if they have fun getting g there. (And you can recycle those skipped scenes for later sessions)

Not sure about good modules.  I think all the published 7th Sea mods were pretty much panned.

Syrneth ruins and pirates?  You may want to have a look at the Explorers Society book from 1st ed – lots of good ideas in there for differentiating the Syrne and sample artfacts.  Pirate Nations has a few things that might be worth converting, and of course a handful of villainous pirates.

Sal's made a pretty good list.  Only thing I might suggest is to sit down with a stack of index cards.  Put one story element on each that you like and want to include.  Then lay them all out on the table and start thinking of connections.  For a short campaign, you'll want a beginning and an end.  Leave the players to chart the course between them.

The real problem with the published adventures is the changes in the world for the new edition. Most of them had ties to the metaplot that is no longer in place. Youight get some ideas from the short plot hooks that were done in the final section of Rapier's Edge.

Oh!  BTW.  This might be of use to you, El.  Its the adventure I ran for my old 7th Sea players.  I don't think it would last for a whole campaign, but it should last last 2-3 sessions easily.  Plus, pirates, syrneth, its all in there.

My biggest advise is have names at the ready. Names for places and names for people. You never know when your players decide they really want to know everything they can about some random NPC that is suppose to be a background character. You can improvise most of that information, but having names at the ready makes it so you flounder less on the introduction to the character.

You don't need to assign any extra information to any of the names, just have a couple of lists of random names nearby. Maybe divided up by countries.

Have them do a 3x3x3. Ravenshadow (I think it was RS - Sal will correct me if I'm wrong) from the old 7th Sea forum suggested this years ago, and it is the most useful tool I've ever learned. Basically, if you're having them create their characters in advance, have them also create 9 NPCs that relate to their character: 3 Friends/Allies, 3 Rivals/Enemies, and 3 "Other"s such as people they admire, folks they'd like to meet, or just associates. They don't need to stat them, just a name and one or two sentances describing the NPC and their relationship to the Hero. You'll be able to assemble your campaign pretty much directly from these characters AND you'll automatically have a way to draw the hero into the story. It'll also give you a really good idea of the sort of story each of your players wants to see.

Leverage breaks. Obviously not too much, but you're going to want to break to get some pizza, a drink, use the loo and what not an hour or two in. This is a good time to stop and go "So what's happening here? Where is it going?" You can spend some time thinking about what's missing and what you might need to take our or add in. If your players decide to go a route you weren't expecting, you can always break for 10 min and put together a few more NPCs or stats that will make sense ro the new plan.

In general, I try to avoid saying "No" to Players unless they're doing someting treally egregious. The more you roll with what the players want to do, the more likely they'll think you're a genius who planned it all from the start.

Don't try to pack in too much. Everything typically takes longer than you think it will, and in the off chance you do finish up early, that's fine; it's better to leave them with a short crisp run that keeps them wanting more than dragging it out. I still make this mistake on occassion (esp when picking up a new system and settimg.) it's hard to judge what Players are going to get caught up on.

When I'm assembling an adventure I tend to think in terms moments or accomplishments with lots of possible ways to get there. For example, let's say I know that at some point the heroes need to get information about where Klara is being held captaive by von Heidelburg. There's lots of different ways to get this information A) Maybe a local shopkeep has seen something supicious while out making a delivery, B) maybe they can find one of the villain's guards, C) perhaps a roving band of musketeers has heard a rumor or two. The key "moment" is that they find out where Klara is being held. Who they get that information from isn't that important...well it could become important, but it's less important than actually getting that info. So I'll come up with a few ideas for where Klara could be held (3 or 4; note that each option changes the various possible approaches that the heroes could take to rescue her) and a few ideas for where they can get the informaiton of her location. I may not use ANY of those ideas during the game because a better option comes to me, but now I have some options depending on how the party decides to go about mucking around that I can get that info to them and help them get to the next stage of the adventure.

Kevin is correct on the 3x3x3 stuff. I plan a break for the halfway point. Outside of that, players can slip off to the bathroom when they are not involved in the action. But I do call them unplanned (normally when I need to think about what just happened that I never anticipated so I can work up a solution).

As Bradley said, you never know how will your players react to what they're going to encounter, if you walk them through a forest they might want to know who lives in it and then, the might want to know everything about that lumberjack you invented on the fly.

Gather some referente pictures, it'll help if you suddenly need to describe a location you never thought you'd need. Browse, if you can, archetype characters (bartenders, money-loaners, peasants, noblemen, clergy, merchants) from any RPG you have in hand (they can help a lot when you have to improvise a new NPC).

It doesn't matter if your players go the opposite direction you planned, they don't know the plot or the locations. Maps can be turned upside-down if they decide to head west instead of east, villians prematurely dealt with could end up being henchmen to more powefull villians, that stolen cargo hook can turn into a kidnapped noblewoman or missing child hook if ignored...

Don't panic! If you need time to think call for a small break and tell your players you need some refreshments, to find your notes on a certain partbof the story or have some fresh air.

Time is relative while roleplaying, your players could sweep through a derelict exploring scene, wich you thought would take them several hours, in just a few minutes or spend several hours deciding if they want to go north or south from their current location. Let them set the pace unless you sense someone's getting bored (random cutpurses or robbers come in handy in those situations) and, if you have been playing for a long time, don't rush important scenes just because lunchtime is near, wait for a dramatic moment and stop the session for the day leaving them with the cliffhanger.

One important thing to remember as a DM: you are not playing against your players but neither you have to abide by their rules. It's necesary to hear their concerns and find solution together with them but you don't have to give them everything they ask for.

And last but not least: have fun! As a DM I find I'm not constricted to play a single character but, instead, I get to play a whole World! Create NPCs you'd want to try out or you think you'll have fun playing.

Edit: when I first started directing what I found most diffucult was to give equal time and rewards to the players. If this troubles you too, have a aparecer sheet with the names of the players and add a mark next to them when their character took the spotlight during a scene. It helps keeping track on how's everyone doing.

A little tip for sudden random. NPCs and how to make them unique. You don't need to have every character drawn up in advance. If the players start showing interest in a character that you never planned on, just base their responses on someone you have met or know. Then make note of who you used so it stays consistent. Need a clumsy servant, maybe he is Todd the waiter that messed up all the orders on your mom's birthday last year. (He probably needs a new name). Troublesome scholar they decide they want info from that you know will be a dead end because they got distracted, maybe it's the tech support guy from T-Mobile who drove you to switch carriers last month.