We've all been in that position before and sadly it really just boils down to playing ALL of the champions at least 3-4 times (to get used to their moves and making the decision you don't like them b/c of their playstyle and not just your inexperience using them).
At least you have your main which you can use if you absolutely HAVE to win the game. Everything else is for testing the water until you get some backup champions. I'd also recommend finding a champion for every role (top, mid, adc, support, jungle), because if you get in a team where no one can play anything but the same role, you're going to be essentially SOL.
Edit: Feel I should mention you should DEFINITELY take advantage of free-week champions to help build your repertoire.