First you need to gauge how deep you are having trouble with sticking to one champ/role. If you're like me and find it really hard to then otherwise it may not be worth your trouble. Apparently ranked isn't all that worth it anyway.
But if need be, to find a main, instead of asking for who specifically, you should set yourself a set of requirements:
1 - Is the champion worth dedicating that much time and effort to?
There are certain champions in which there is a pretty low skill floor and ceiling such as most of the 'intro' champions like Yi, Annie, Garen etc. The main reason you'd want to main someone is to master the micro-mechanics, not the macro-mechanics. When people say "Yi/Annie/Garen aren't easier to win with cuz you still need to know about map awareness" or some bullshit like that, they're saying it for the sake of macro. That goes for every champion to ever exist. Don't mix up micro and macro. Champions like Yasuo, Riven, Draven etc, with godly micro-mechanics can be incredibly rewarding.
2 - Does the champion have a high ranked presence?
There's no real point in trying to main someone who is either always picked or banned for self-explanatory reasons. Take Morgana for instance. In this CC/hook/engage heavy bot meta, playing Morgana/maining Morgana seems like the perfect move, but because it's such a perfect move, she's always banned out.
3 - Can you genuinely find them fun?
If you're gonna be maining someone, you will be dedicating SERIOUS amounts of time to them. If you don't or can't find said champion fun, then maining said champion just won't be sustainable, regardless of how effective your mechanics are on them.
4 - Are they grossly overpowered or grossly underpowered?
If someone you really wanna main is either of these two, chances are, you're gonna run into trouble since champions in both categories are always subject to buffs, nerfs and potentially even reworks. For instance, maining someone like Akali back when she was released would've been a good move at first, but of course now we know how overtuned she was and how many nerfs she's required (or not required, but still received). Now look at Ryze. No one can really fully learn/master him because by the time you have gotten halfway through mastering him, Riot would've issued in his next annual (or even biannual) rework. What about for instance, Orianna. She would be a perfect 'main' champion since she's really a champion whose effectiveness scales 1:1 with the player's skill. She's never overbearing or underwhelming in any meta and has stayed pretty stagnant with her numbers. Of course now we know the only reason why she received the Shockwave buff is due to Worlds but that's an outliar of Riot wanting more viewership and highlights than Orianna actually being weak or not.
And finally, if you're like me: 5 - Do you like the way the champion looks?
I dunno, I can't play any champion I don't like visually but that might just be me
But yeah, just a few things you should keep in mind when trying to find a main. I encourage you to not just follow suggestions people give you without running through some of these points (or all of them).