It's actually really easy.
Just pick the champion you enjoy playing the most in the role you enjoy the most and feel the most comfortable at playing.
Don't look at meta or stats.
Choose the champion you can play over and over without getting tired of playing him / her.
For me that's Fiddlesticks, because I love the champion and I can play him in multiple roles (jungle, mid and support). ^^
In addition if you didn't find a champion like that just try to play some games with other champions, you can use the free champion each week for it. I found mine Taric when i tried to play him after he was reworked as i was support anyway.
There'll be as many answers to this question as there is summoners, and one can only tell what their own experiences are with the game (obviosuly), I took my liking to the unusual picks that were forgotten in the meta, so I was basically playing every champion there was, until I found someone I really liked, and in Season 3 to 7 it was Shaco, and now it's Garen for me :)
Just try some champions in the rotation. I tried Trundle ones and I felt most comfortable playing him. I was trying many new ways to play Trundle and now it's my main.
Sometimes you don't always have to play the champ. Watch how pro players use the champ and if you like it go on! But to feel it yourself you have to play of course.
Narrow your search of over 100 champs by asking yourself what playstyle you enjoy to play the most: tank, assassin, adc, burst mage, skill shots, etc. Then look into the champs that closely match your criteria. That is a good place to start. Another is to always play the free champ rotations so you get a taste of everything. Eventually you will find a main and then you will find a new champ that you like better. I have gone through about 5 mains in 5 years playing.
Also, you can try to point out what you like in your current favourite champions (role, roaming potential, skill cap, if they are early/late champions, etc...), than point out what do you want your main to have, put that on a paper and search for a champion that matches *** much as possible with that.
Mainly it's experimentation. Riot makes champs free to play for a reason. Take the time to try out the champs that are in the rotation exery week. Start by figuring out what role you'd like to play, either support, mid, top, bot, or jungle. I recommend picking your second favorite too, as a backup for draft picks. That will help you restrict the champion field. From there, you can decide what champion you want to use. Like I said, use the free champion rotation to your advantage. That way you don't spend your hard earned BE on a champion you aren't going to play.
If you need a main for low elo ranked, look at how easy the champion is and how powerful they can be (Syndra, Annie, Miss Fortune, or Chogath for example). If you want a main for casual, just play a mix of champions when they are free to play and pick the one that makes you either have the most fun, or you play the best. Practice a few champions a lot, and then pick one that you like the best :D
Unless you want a main for high elo ranked play... Then you gotta pick something with high skill= high damage (and or play making potential). Champs like Zed, Yassuo, Riven, Draven.
Just pick the champion you enjoy playing the most in the role you enjoy the most and feel the most comfortable at playing.
Don't look at meta or stats.
Choose the champion you can play over and over without getting tired of playing him / her.
For me that's
Unless you want a main for high elo ranked play... Then you gotta pick something with high skill= high damage (and or play making potential). Champs like Zed, Yassuo, Riven, Draven.