Maining a champion.

Think about your tendencies (Protect the carry? Split push? Assassinate?) And go from there?

Chances are if you're constantly bitching about your "team" not killing carries or bitching about your "team" not pushing enough, it's a sign that role is for you.

Main the champion has the best lore for you.

Choose based on what champ has the most skins you like and buy all the skins.

I personally like the style of certain champs. I like Nidalee because she's like a Druid Assassin type. I like her lore and I like her look. And she's fun. I don't main her but if I ever were to have a main, it'd be her

I am a Yi main with secondaries as Fizz and Hecarim. I am a jg/top main and love playing mobile bruisers with strong late games that can take down ADCs easily while still being very tanky. High damage autoattack bruisers are the closest you get to an "all in 1 package" champ in league; they neither are glass, nor do they hit like wet noodles. Yi is the ultimate ADC killing machine, being one of the few melees (the other is Fizz) that can kill an equally fed Kalista and Vayne 1v1. Not even Zed or Jax can claim that. I play Hecarim because he can tank for the team while still being a huge threat due to his mobility, initiation and high AOE damage that can shred tanks and squishies alike.

Maining a champ comes down to who you think feels the best in their mechanics. Some like the smooth Yasuo sliding chase mechanics, others like the staying behind and impaired mobility that Xerath has when poking from insane range. Some like combo's in their abilities that reward with extra damage like Vel'Koz, while some want to have the bonus of stacking numbers for amazing damage late game like Nasus/Veigar.

Melee or ranged, caster or autoattack-based, AD or AP, glass cannon or an unbreakable wall of flesh, you'll find your champion! Just watch the old champion spotlights and see what you're attracted to

I started off with Katarina main (still my best champion by far, hardly ever do poorly anymore), then moved to support Zilean main, then Nami support for about 3 months, now it's sort of turning into middle Vel'koz. Like the others have said, just find someone who is really fun to use, even if you are terrible at them. If you enjoy them, use them often to learn how to get even better.

I have a couple main champs, and try to pick them whenever possible unless I'm heavily countered or something.