I got into a bit of trouble hitchhiking when I was 17 despite having my mom's permission. I was out of state. definitely stay under the radar and be prepared to give some pretty detailed fake information ( a cousin, perhaps?) qualifying you as an adult.
you should think twice about how you're going to survive. cities are rough places for the inexperienced. I'm going to give you the benefit of the doubt that you have ample reason to leave, and maybe you'll only be gone for a few months and then come home. but you should just remember that meeting really basic necessities can be difficult without a job or a place to sleep. hell, I do it all the time but I've been working at it for awhile.
A few pieces of advice that you probably dont need but are worth repeating:
don't rely on other totally shady homeless kids to take you in. you will pick up their habits and they're usually bad ones.
stay the fuck away from hard drugs. this shit will either kill you or make your life miserable. don't touch them. my heart breaks a little every time I see a teenager strung out on dope and its just a really bad scene.
always have condoms. you're approaching an age (maybe you've already made it there, who knows) where people will be moving quickly from french kissing to just straight up doing it. you never know when you're going to jump in the sack so be prepared for it. pregnancy and diseases will really put a damper on your escapades.
good luck duder. let us all know what you decide to do and how it works out for you.