Welcome to MxO!
In my expereince the best roles and one ones you enjoy the most are the ones you make for yourself. To help with that I'd check out this thread for a somewhat accerate overview of all the organizations and sub groups. If you can't decided from just that then I recomend finding someone in the org or rogs that intrest you and talk to them about it.
Once you've settled on an org to work for this list can help you find a faction you may want to join.
From their its all up to you on how you want to play.
