Let's face it. We're whupped. Unless we've been following Star Trek all these years or have the free time and disposable resources to acquire and watch all five series collections and the movies, we're whupped. Unlike Star Wars, which has comparatively little face time on the screen and requires a more generous approach to printed, EU, canon: Star Trek's only canon is what has appeared in the various television series and the films. And there's a lot of it.
Worse, we're captains in Starfleet, or from the Klingon Imperial alliance, so if we're roleplaying Academy bred captains and captains alone we can't exactly pull a Luke Skywalker and plead ignorance of that big old confusing universe out there.
There's simply no way we're going to be real experts on the setting.
But the beauty of the thing is that we can fake it enough. Even the details have a single repository we can keep on hand and refer to as needed. Generally, the story dynamics of the series and ideally the best roleplaying will have more to do with concepts and personalities rather than the nuances of Federation law or the history of the Telerites. The who? Yeah, well, yeah. I could have said Vulcan but, dude, there's alot of people in The Federation and outside of it. STO is set in the future, long after Nemesis - the last Next Generation film, and even The Ferengi have joined up now. The Klingons have absorbed The Gorn and The Orions and the Nausicaans.
Bogglin'? Of course you are! Either you know what those names mean and can't imagine how this has happened or you've never heard of them and aren't even sure you want to do your roleplayer's due diligence and research them. So many names! And that's scratching the surface.
For people who already know Star Trek I'd recommend keeping up on the Timeline as it progresses on the Cryptic website. It should explain how we got from here to there:
For those who don't, there's one great free wiki resource for canonical information organized by subject:
Memory Alpha's articles may link to Memory Beta, which is noncanonical additional information but keep in mind Star Trek really means noncanonical - this stuff is for entertainment purposes only unlike Star Wars EU content which is often treated as essential. It also links on occasion to Ex Astris Scientia which is another fan run site and which contains canonical and noncanonical information.
Next we end up with Printed Material:
As always the resource of first resort should be the latest edition of the latest version of the tabletop RPG about the setting. In this case we're looking at Decipher's Star Trek Roleplaying game. While the rules will have nothing to do with how Star Trek Online will play the "fluff" text should help you learn, or refresh yourself, on ideas about the setting and the technology in general. With this in hand you'll learn all the essentials you'll need.
The Player's Guide covers the basics of gear, the nature of The Federation and Starfleet. It discusses a bit of what life on a Federation starship is like. You'll also find ideas to help you brainstorm a character. The focus is a bit broader than one might imagine, or generally even want, as it covers many non-Starfleet or military approaches to character creation. The information on individual alien cultures is minimal but you can always do homework on Memory Alpha or with the Encyclopedia.
The Narrator's Guide covers more detail about the setting, historical detail, possible kinds of stories, the nature of space and how to approach visualizing a campaign and planning for it. There is useful information here but it's not as fundamental as the information in the Player's Guide.
The Starfleet Operations Manual will be very handy for Starfleet characters. Diagrams of a ship's bridge and a tricorder are there for your investigation. Descriptions of alert states and standard operating procedures for away teams or other situations are described. Many additional Federation races are written up briefly (did I mention Telerites?). While the list of ships isn't what I'd call comprehensive it does cover the important Federation classes along with what's in The Player's Guide and The Narrator's Guide. The nature and function of outposts and starbases is also described.
And that's all you really need. Those three books will give you an idea of what a normal Starfleet Captain might know, at least in part, along with plentiful ideas for roleplay and the rest can be researched ad hoc in Memory Alpha.
However, for completists who really want to fool the Trekkies, I'd recommend the following as well:
The Star Trek Encyclopedia is essentially a "Memory Alpha" that's a book and you can use it to research words or concepts on the fly as you roleplay. Someone drops a reference to the Lissepians or "osmotic pressure therapy" in a line of dialogue? Don't tab out and lose your place! Crack open the Star Trek Encyclopedia!
Okay, you like Scotty. You really want to be able to drop complicated technical terms on the fly and do it well without having to take notes on over a thousand dollars worth of DVDs. Hell, you might just plain be curious how teleporters or warp drive works! You want Star Trek: The Next Generation Technical Manual. While it doesn't have detailed maps of the Enterprise it does describe all the basic tech which applies to almost all Starfleet vessels. Keep in mind, though, there is some new technology which has come up since the Borg arrived and was tested in the Dominion Wars. Ablative armor among other things. I suspect that information will be in Star Trek: Deep Space 9 Technical Manual. The TNG Tech Manual covers more subjects more broadly and has more general utility for our purposes. The rest can be dug up on Memory Alpha as needed.
You like Data and Spock or perhaps The Doctor? Star Trek Science Logs may be for you. While it doesn't go into great detail explaining individual events or phenomenon it does look at sci-fi subjects that have cropped up on the shows and looks behind the scenes at real science and speculation around those subjects. There's another book along these lines, The Physics of Star Trek, which I haven't looked at yet. It appears to be a more serious look at where Star Trek goes right and wrong with heavier real science content. More useful to the roleplayer, from what I can tell, are the Science Logs as it could spur some interesting roleplaying ideas with a much broader survey of what's come before.
Oh, you're more of a Sulu or Geordi junkie? Big on the helm and on where the ship is and where it might go next? You'll want to look into Star Trek: Star Charts. This is a pretty neat book for sorting out in your mind where, say, The Romulan empire is in relationship to Cardassia or how a starship captain orients himself to where he is. Sector 001 may be Sol, and what exactly is a sector anyhow, but the center of the galaxy is the real middle of things and all four quadrants are split up around that. Are we heading spinward or coreward or rimward? What's in the middle of the galaxy? What bounds it? I know what an M-class planet is. What are the other classes? What's a "main sequence" star? I do put Star Charts last on my list because the basic information here can be gathered from other sources including the Decipher Narrator's Guide or Memory Alpha or The Encyclopedia. It's likely that STO won't be all that fussy about everything fitting together based on this, canonically derived but non-canonical, book so its utility may be limited. Still it's pretty and it will help you figure out "You Are Here" when you get lost.