I'll say what I think is the most comprehensive advice:
Play the different playstyle champions.
I.E play aggressive mechanical champions like zed, lee, riven, yasuo, etc. after playing a lot of them, like 20 or so games, see if they aren't getting old.
then move on to the next playstyle.
Don't focus on roles/champions so much as the playstyle that fits you the most. Then, get more specific, the ROLES that fit that playstyle how you enjoy it, and what roles allow you to do the things you want to do. then look for your favorite 3-4 champions.
For example, I started maining midlane recently and I want to play all rounder champions with a few really high mechanical champions that I'll play most of the time, and learn a bunch of champions that are really well rounded.
**Mains ** (will default to these most of the time, unless their comp doesn't let me do it easily)
- Riven - early-mid game AD mid (lane winner, like yorick)
- Azir - mid-late game AP mid (doesn't matter what their comp is, or yours, you WILL carry if you are playing better)
- Yasuo - mid game AD mid (let the bodies hit the floor if they have little hard cc) KILL EZREAL. DIE YOU BITCH.
**Secondary ** (if they have lots of cc, or an easy way to dive on me/pressure mid lane)
- Cho - mid-late game safe AP mid (safe, just farm and push lane, consistently useful)
- Katarina - mid-late game AP mid (katarina is pretty safe, and if i have team with good teamfights ill pick kat)
- Ahri - mid-late game safe AP mid (against less mobile teams)
- Cassiopeia - mid-late game AP mid (if they are all really tanky, ill just shred them, they won't kill me with E sustain)
- Leblanc - early-late game AP mid (if their mid requires a snowball, lets me shut them down and be useful)
Third (if i need to blow someone up with talon easily and theres a AP top/jng, or if my lane lets me farm very very easily)
- Talon - mid-late game AD mid (for 100% blowing up 1 person, new zed.)
- Ori - mid-late game safe AP mid (if I can farm without backing and good teamfight team)
- Brand - early-mid game AP mid (if they have a melee mid and i can farm)
- Veigar - late game safe AP mid (if I can farm and team is pretty decent late game)
not in this meta: Zed - early-mid-late game safe AD mid (Zed is almost unplayable now above gold with all the safe ADC's and CC comps)
my mains I know 95%, 2nd i probably know about 70%, and third picks I know about 60%
they are all good enough to play them in diamond, and it lets me play my desired playstyle (heavily mechanical damage dealers that have no kryptonite)
I would like to add akali to this but sadly she has a kryptonite, pink wards, unless you get to end game build and you have 50% spellvamp with 4000 damage sustained burst mobility abilities.
So this is just an example of a really fleshed out and LARGE champion pool and is built around a certain playstyle and is viable in pretty much every meta, if they have lots of cc then i play a safer laner like cho or ahri or brand/ori, and if I can farm i pick hyper scaling champions, if they pick vulnerable laners or I feel confident i can kill, then i pick my mains, or i just pick azir and can do everything all the time without a problem.
Find whats the most FUN, and doesn't get OLD. then get specific, after getting specific, you flesh it out and fill weaknesses of main champions with the strengths of others.