This topic gets made like every 2 weeks, so I'm making it now save everyone the trouble.
GENERAL ADVICE FOR PICKING A MAIN:
1. watch videos of good players using a character, see if the general gameplan they use is what you want to do.
2. look up combos for the character, and try them out in training mode.
3. play with the character yourself, see if you like using him/her/it.