But you're not sure where to begin. Well, things are different than at the end of Wraith, you actually have choices of where to start! There's 4 raids and 5 bosses you can start with, and having downed all but one of them I sat down and rated them from easy to hard. Obviously this is my option, but at least it'll give you some ideas.
1. Pit Lord Whatshisface->Located in Baradin Hold, the Pit Lord is only available if you have Tol Barad. Remember VOA? It's like that. Honestly, I think this boss is the easiest to start with. The strategy (divide into two groups, stack on the two tanks, run around when there's fire on the ground) is very simple, and the fight is more of a DPS race than anything else. If you can't take this guy down before the enrage timer with no DPS deaths, you need to look hard at your DPSers and make sure they're pulling their weight.
2. Magmaw (aka worm dude)->This guy's located in Blackwing Descent, and he's a big lava worm. There's quite a bit more going on in this fight (lava parasites, floor on fire, boss eating a tank, having to use chains to pin the boss's head down, a burn phase), but once you get the hang of it, you're golden. Keep in mind that there's loads of ways to handle the parasites, and experiment.
3. Omnitron Defense System->Also located in Blackwing Descent, these guys are a bunch of robots. The robots activate and deactivate, they all have different abilities, and once their shields come up, any DPS can cause a wipe. There's much more going on in this fight than in Magmaw's, but if you lack enough ranged aoe or a good kiter for Magmaw's parasites, this is a good option instead. Also, even if you off Magmaw, you still have to kill these guys sooner or later to progress in Blackwing Descent.
4. Conclave of Wind->This is located in Uldum, and is off the coast. You're got 3 bosses, and all 3 need to die within a minute of each other. It's not a hard fight, but is very unforgiving of mistakes or bad luck with the RNG. Also, killing these 3 only unlocks your access to Al'Akir and he's probably not one you want to tackle until you've got better gear and experience.
5. Halfus-> Located in Bastion of Twilight in the Twilight Highlands (look for the spiral tower thing), this is an extremely variable fight. Depending on the drakes you get, it can be a piece of cake or your worst nightmare. I'd say check to see what drakes are up, and then decide whether you want to tackle it or not. My raid team is actually saving our lockout with Halfus down so we can try the twin dragons (almost got them) without having to deal with a really hard drake composition. Plus, the trash before Halfus is rather icky. But if you want to progress in Bastion, you have no choice but to kill him.
If I was starting a raid team, I'd start with Pit Lord, then (or if no TB) hit Magmaw, go see what drakes were up on Halfus and maybe tackle him, and then tackle either the Defense Council or the next boss in Bastion. But I've really gotten to dislike the Conclave, so that has something to do with it. No matter what you decide, remember to do your research ahead of time, try different strategies if something's not working, and adapt as needed. Raiding in Cat's harder than Wraith was (imho), so be prepared to spend quite a bit of time on the cold floor dead. Best of luck!