Hey RedBaron, welcome to DW!
Also, make sure that you're at least 80% familiar with the general rules for combat (don't worry too much about things like Grapling and Non-lethal combat. Focus on the basics: standard attack, multiple attacks, charge, semi-auto, full-auto, move actions. You can always look up special rules if necessary but these basics should serve you for your first session.) I would also make sure to understand any abilities that the enemies and NPCs will have.
I would also give your players a bit of homework (if possible) before their first session so they understand that they have a responsibility to the game and that not everything falls on the GM's shoulders. Make sure they are familiar with the rules for tests and combat as well. They should read up on the rules for Fate points. Also, they need to read the description for any skill or talent their characters possess so they know what they characters can do. (Basic rule of thumb: if your players aren't familiar with it, then don't introduce it all at once. It makes more sense to learn the foundation first, and add more layers later. Along those lines, I would consider skipping Cohesion and Squad Mode for the first session or two and intorduce it later. I might stick to non-librarians and non-techmarines for the first session or two as these characters tend to involve more rules than 'regular' marines. However, if one of the players really wants to play that Librarian, then make it the player's responsibility to learn the rules applicable to their chosen speciality.)
I would tell you to make sure you've read the adventure from start to finish. Then, take a few minutes to write some notes for yourself about some "What ifs"? (Much like Adeptus-B said, it's unlikely that your characters will get everything 'right'. It's more likely they'll go in at least some unexpected directions. Also, if you and your players are new to RPGs in general then you might want a way to get the adventure 'back on track' if the characters are overwhelmed by the freedom of "So, what do you do now?")
Think about it like this:
- "What if the PCs don't ask that NPC the right questions and therefore don't get some crucial information they need? Solution: one of the PCs (test INT?) remembers some notes about this in the mission briefing. Or, maybe they find a clue elsewhere - or even another NPC - that reveals more or less the same info?
- What if the PCs want to go left when they should go right? Solution: they encounter a group of weak enemies. On their bodies they find clues suggesting they should go right. (Or, you just carry on as if they were supposed to go left all along.)
Most important tip: don't take it too seriously and just have fun with it! If your players are new at this and you are too, then everyone should be okay with people making mistakes, things not working out as expected, etc. If you can't find a rule, then make it up until you have a chance to look it up. So long as everybody (GM included) is having fun, and you try to be fair, then you're doing it right.
Let us know how it goes!