You're level 30, you should have played enough games to know which champions you enjoyed until now. Pick them.
I started to play the game back in season 2, right after Volibear was intrduced. I started with GP, because he's a fucking pirate. At level 15 or so, I found Shaco. His kit really fit with my playstyle and I've mained him since then. I've played so much of the champion that I can play him at any role (except adc, as no team has allowed me to do so yet) and suceed at least in lane.
I also liked Gnar the second he was released, and as I enjoyed his kit, I bought him and turned him into my second main.
Those are my only two mains. Shaco is by far my primary main and is probably the reason I still play League of Legends. I use him for Mid and Jungle. Gnar is my secondary main, that I use whenever I'm not confident on the match up I'll have on top lane, as I know I can come back with him even if I'm 2 or 3 kills behind.
They are the bulk of my champion pool. With that said, I'm also reasonably good with Brand. I picked him up because I like to blow things up and can't play Ziggs well no matter how much I try. He's my go to mid if I know the enemy laner would destroy my Shaco (for example, an Annie).
Darius is my second main top, mostly because I love his brutal playstyle. I played him once pre rework, got into a 2v1 fight against the top laner and the jungler. While I fought both of them, mid decided to join. I killed both. When mid arrived I was at low health, but had the time to do one more combo. I died, but the bleed got him. Since then I started to enjoy the shit out of the champion. When I can't trust on my team to follow my initiations as Gnar, this is what I play.
For top I also play Mundo, Singed and more recently Swain. Because I like being beefy and surviving things I shouldn't be able to.
Whenever I get stuck as ADC, I usually pick a bursty one, as Lucian, Graves or Ez. I do that because I had a great time with assassins, and being able to quickly chunk a massive part of the enemies health bar feels good.
If you pay attention, you will notice that most of my pool consists of champions I enjoyed playing with, not champions that are meta or strong on their own. All you have to do is play enough games to find out what's your playstyle and which champions fit well into it.