First off, here's a link to a helpful video from Omni that talks about how to find a main. His video is more on the context of Super Smash Bros., but the concepts still apply to some degree in League of Legends, and can be really helpful in helping you find a main:
https://www.youtube.com/watch?v=d1UAfFbLvLc
How I found a main is that I tried out a lot of different champions and saw which champions fit with how I wanted to play the game. In particular, I found that I liked a lot of tanky champions that didn't deal a whole lot of damage and were good at enabling the rest of my team to succeed. In addition, I also enjoyed the jungle a lot because it had a strategic element that wasn't found in a lot of the other roles. That's what drew me to Nautilus because he was both a tanky champion with lots of crowd control and was (and still is) a good jungler.
You have the advantage of playing with a group of friends because they can give you advice on what they remember you playing as. Ask your friends for advice on what you should main based on what they have seen from your play on the champions you have played.