Good advice so far.
Be proficient in at least four of the five roles, and be sure those four include support and jungle. Know how to play at least 3 champions well for each of those roles - more is better. Know how to play at least 5 champions well for your preferred roles, and be aware of their good and bad matchups. More is definitely better here.
Have a large selection of mastery pages so you can adapt to as many situations as possible.
Have at least four rune pages, one AD, one AP, one Jungle, and one Support. More is better here, too, as it gives you increased flexibility.
Have several hundred normal wins under your belt from draft normal games. This will give you the experience with the types of situations you will encounter in ranked play, from champion select, to laning, to midgame objective control, to endgame pushing and teamfighting. The more different situations you have experience with, the better you will be able to react to them in ranked.
Also, work on your communication. Work with your teammates in champion select to avoid conflict. Don't rage in games, and don't respond negatively to other players raging at you. Tensions get pretty high in ranked games, so be able to keep your cool. If you can't do that, don't play ranked, because a team fighting amongst themselves has already lost the game.