I'm currently 18 years old, and trying to open my own sneaker Boutique. I just don't know where to start? Should I first look for a spot or worry about where I'll be getting my supplies (shoes)?

I would go to work in a shoe store, Get some experience first. Getting paid while learning is a win win. Where are you going to get the money for inventory? You will also need permits, license, fixtures, several months rent up front, money for advertising etc. Contact www.sba.gov they have a lot of information that will help you. Then make a business plan, which is your guide to how your going to start and run your business.
Good Luck !