I think this is the 40th time I'v answered this question since 2001.

First you get a tank: pro pally or whatever

Then you get a healer: resto shaman or whatever

Then you get a dps: mage or whatever


You will find others to group with very easy with those three as you are basically gtg off the bat, finding a healer when you have 3 dps and a tank or finding a tank when you have 3 dps and a healer is a lot harder then finding 2 dps .....

You have your core and now you can add more dps as needed.