not playing ranked till they do ranked teambuilder
Good. You shouldn't play ranked if you don't have atleast 1-2 champs per lane that you can play. And even if you can't, it's fine. But atleast have the decency to queue dodge whenever you are forced to go that one role you don't want (if there are multiple roles you don't want, then you probably better just don't play ranked as you'll lose too much LP to climb properly, although in the long run you'd still climb).
My advise: Just look for some easy to play high winrate champs. Champs with high winrate usually do well even if you don't play them at their full potential. You can play Janna for example and mess up a lot and still be useful. Of course you won't be near as useful as a Janna main, but a lot better than if you'd play Thresh or Bard with the same (low) amount of experience.
Note that if before the game starts, you predict that your winning chance is below 50%, you'd better queue dodge because you'll only lose LP that way and no MMR. MMR is what matters in the long run. Your LP will adjust to match your MMR but it takes time.