I have to say this is a bad idea, I know it probably sounds really great to you but trust me, it isnt. I ran away when I was 15 and got caught by the cops while I was crashed out in a friends attic a week later, then more recently (like this december) I did it again and ended up homeless wandering the streets with a friend for a few weeks. During that time I got arrested and taken back home, a week later the friend I was with ended up in jail. Trust me, a lot of seriously fucked up shit can happen even if you think your plan is solid. And what about school? Be realistic, even if you somehow manage to make most of your income off of selling drugs, you will still need to fit into society in some ways...you really do have to go through highschool and all.
Why cant you just be satisfied with going out a couple times a week and getting trashed with your friends? Wait til youre 18 mate, it will save a lot of trouble...