Get Deadly Boss Mods and voice chat. Not every Raid Finder group will use the same addons and tools, of course, but you can expect most groups will want you to have Deadly Boss Mods. Yes, there are alternatives. However, Deadly Boss Mods still seems the most popular go-to. When a raid leader asks you to get a different mod, you might as well. But have DBM installed before you hit the Raid Finder button.
Voice chat is a little more difficult, since there's a much wider array of available voice chat clients. Still, you should have Ventrilo and Mumble already installed. I prefer Mumble, personally, but I encounter Ventrilo among PUGs much more frequently.
While nothing in Blizzard's official tools demands you have these tools, the random groups you encounter will expect everyone to be DBM and voice chat ready.
Have food and flasks. Remember all those annoying mods that do things like whisper "Joe-Bob ain't drunk his flask"? Well, they're back -- and probably with good reason. While Raid Finder difficulty is easier than normal raiding, you still want every little numerical advantage you can get.
More than a few Raid Finder groups will have incredibly generous folks who provide cauldrons and food for everyone. But don't bank on that happening. Try to have your appropriate buff food and flask ready to go.
Be polite and patient. Being polite and patient with your group probably sounds like silly advice, but what blew me away about Raid Finder so far is that groups are pretty cool. I've found the stress level to be much lower than even 5-man dungeons. I couldn't tell you why that's the case, but I haven't encountered anyone who's really irked me so far.
So with that fresh, exciting opportunity out there, do your best to be polite and patient. Remember that this is new content, that raid groups are learning, and everyone's (presumably) doing their best. If you find yourself getting frustrated, grab a cup of coffee and dial it back a notch.
Watch videos before you queue. The bosses in Dragon Soul via the Raid Finder aren't exactly difficult. I blew through my first Dragon Soul in less than two hours. That being said, it helps if you are forearmed with a basic knowledge of the fights.
Make sure you check out our guide to Dragon Soul bosses in 5 seconds or less. That'll give you a basic idea of what the heck's going on. Really, though, you should still survey some video guides to get the in-depth why and wherefore of each fight.
Yes, the fights are perfectly explainable by a competent raid leader -- but that really shouldn't be necessary. Hit up that 5 seconds post. It'll cover the vast majority of what you need to know.
Be prepared to chill out. The harshest part of Raid Finder is that you end up with 25 opportunities for someone to disconnect, go away from keyboard, or otherwise wander off into unknown territory. This is a natural consequence of getting a large group of humans together.
If someone leaves the raid, you'll have to wait for a replacement. I've generally had to wait longer for healers than anyone else, but you definitely have to wait. Combine that with all the usual raid breaks, and you could find yourself killing some time.
Play a movie in the background. Bring a book. Make sure you can occupy a few odd moments without losing your mind. If you're going to freak out over five idle minutes, then Raid Finder is not the place for you. Remember how I said it took less than two hours to all four current bosses? Probably 35% or more of that was spent waiting for replacements. It's going to happen, so prepare for it ahead of time.
Visit the WoW Rookie Guide for links to everything you need to get started as a new player, from how to control your character and camera angles when you're just starting out, to learning how to tank, getting up to speed for heroics and even how to win Tol Barad.