I am 17 and I want to start a freelance video editing company, using Motion and Final Cut Pro X. Only problem is, I have no idea how to go about starting it. Where are some good places to begin?
Depending on where you live, there will be some specific steps you need to take in order to start up your own business (which you'll ultimately want to do if you want to start charging people for your services).
Other than that, I'd recommend that you start out by collaborating with some kind of non-profit organisation, if you can find one that have a need for the kind of work that you offer. This way, you'll get some exposure, you have something concrete to show to potential clients, and you start to build a network. Next thing, try to land a job where you at least get paid something; to get paid, even if it's just a really small amount, is a big step if you've never received payment for your work before.
From there, just try to increment the amount you're getting payed every time you take on a new job. As long as the increment is small enough, you should improve fast enough to justify the new pay-grade.
One good place to start would be the US Small Business Administration. You're going to have to file for a business license, pay taxes, learn about the law, consider hiring employees, etc. This website has information on all of those topics and more. You also might want to look into small business classes at your local community college.
I'd recommend posting this same questions on the Startups Stack Exchange. There are a lot of people on that forum who will have great business advice for you!