How do you start a contracting company?

Hi,
You have a good answer above, I would also add that you can start off doing small jobs for friends/family, maybe advertise on a local services page like craigs list or angies list (with your parents permission, depending on your age). Then you can build some references and have some pictures of projects you have done, this will help you to eventually start a formal business, website, get business cards and start a small business.
Good luck!

My wife works in construction and we know a few people who are contractors/home remodelers. I'd suggest talking to someone who has or is working on a repair or remodeling job at your home if possible. I'd also contact reputable contactors in your area to see if they need a helper so you can learn the job from the bottom up as well as being able to ask them questions on lunch break or riding to the job.