As someone who is level 30, but hasn't started in ranked I have some advice/thoughts.
First - just because you are level 30 you don't have to go into ranked play immediately. You have the option, but you can keep playing normals and working on your decision making and champion mastery. This is what I'm doing rather than jumping into ranked.
Second - One thing to remember is that you have to have 16 champs if you want to play ranked as that is the minimum required to draft. This is based on the possibility that you'd have last pick and all 6 bans, plus the 9 people ahead of you all match a champ you have. 16 is the minimum number required to prevent you from being unable to select. (I have been told that free champs are NOT available in ranked games.)
Third - be prepared to "main" a champ or a lane. In most cases its best to choose 1 champ you plan on playing whenever you can. (not one that is banned often.) Behind them I'd have 2 champs for the same lane you are good with and then 2 other lanes you have 1-2 champs in you are comfortable with. This is based on how ranked solo works. You'll be in a room with 4 others you don't know and each will have preferences. It's possible you won't get your preferred lane, and its best to be able to play well in another rather than being a "mid or feed" troll.
Fourth - feel free to ignore all my advice. I've based it on my own experiences after talking to other players and listening to friends who have gone through their ranked promos. One finished just a week ago and is now in Bronze 2. He asked me yesterday when I would and admitted that my plan of waiting until I felt I was able to carry a team when necessary might be a good idea. He seems happy to have gone for rank, and I have another friend who should shortly. (he's level 29 right now, but plans on entering as soon as he is level 30.) Each person has to make their own decision.