If you don't know anything about ranked, I would probably not play for a bit.
Play normals and practice 2-3 champs (preferably ones that aren't picked/banned every game) for each role, including support. Practice your support ones first because support is probably the only role you will be FORCED to play. Last pick? Support. Even as last pick, you are pretty safe with a comment like, "top is my worst role". People will usually accommodate for you. If you say something like, "i suck support" or "anything but support", well to bad you will be supporting anyway. Anyway, once you are really confident with things like CS and teamfighting on 2-3 champs per role, then you should be in a good place to start ranked. You will probably start in bronze, but that is OK! It will be hell trying to climb out, but once you finally do, you will be happy. Just understand going in....there are many people that troll in ranked. Why? Idk. Probably because they get picked on in school and rito doesn't actually punish anything except "omg rito he said bad word". So be prepared to lose a lot of ranked games because of afks and people playing a champ they have never ever ever (taylor swift voice) played before. Good luck!