Well, the answer is play a bunch of champions. Try getting a few games on each. See what you love about each one, and what you dislike.
Then, make a list of things you really want a single champion to have. Research as many champions as you can and get a small pool of champions that meet those criteria. Then weed them out until you have your main(s)
My main is Taric. I was playing a ton of different champs early, and decided to figure out what I enjoyed. I tried every position a bit, finding support was my favorite. Playing champions, I realized I liked melee champions (from my time in top lane), supports that can heal AND shield, an most importantly, had a good form of cc.
For me, that left only a few champions, and the one I decided on was Taric. He didn’t have a reliable cc against master Yi, one of my criteria, but it he had everything else and at least some ability to deal with him.
I’ve really come to love Taric. Sometimes I get bored and play another champion a bit but whenever I learn how to improve I go straight back to him. He’s just got everything I enjoy about league, and I judge a lot of other champions based on how they compare to him. He’s just truly Outrageous
Hope this helps. Remember: a Main is someone you play the most, not the only champion you play. You’ll burn out unless your Anniebot in disguise or something. Enjoy yourself!