Put yourself in this scenario: you're the number one player in the world winning every tournament with ease. What character do you see yourself using? Which one would you be proud to see yourself with?
Pick a character, practice with them, play against CPU and people and see if you can get comfortable with them quickly. That is most likely the one you will have a good time putting your effort into as you can see improvement not too far away. If not, ditch them and move onto the next one on the list and repeat until you have a main. If you're still having problems deciding on just one, try to use two characters and put time into both. Sure, it will take longer to learn two whole sets of matchups, but it will end your dilemma the quickest and you can start practicing sooner. From there, you may put time into both equally which is fine or put more time into one and get bored with the other. Thus, you can drop the neglected character and voila! Your main is decided.
If you are too lazy or really, really indecisive, just go random and go with the first one you get. After that, stick with that character and have the story of "I only chose (insert name here) because it was random." If you don't like the character you get and are happy with the next one, that is probably the character you will enjoy playing.
TL;DR: Experiment, use multiple mains or go random and stick with it.