The fact that you are asking this question at 15 already puts you ahead of where most people are when they finally decide to change. Most people wait until the consequences get bad enough that they have no choice. You are choosing before that happens. That matters.
Here is the most honest thing anyone can tell you: look at who you spend your time with. The phrase that has stayed with me is this, show me your friends and I will show you your future. No one your age becomes a problem on their own. It happens through the people around you, the situations they pull you into, and the decisions you make in those moments without thinking ahead. Changing your life almost always starts with changing who you spend your time with.
Get back into school if you have stepped away from it, and stay in it if you are still there. Your education is the single most powerful tool you have access to right now and it costs you nothing. Knowledge is something no one can take from you. Talk to your school counselor. That is not a sign of weakness. It is exactly what that person is there for.
You are going to be tempted to go back to what is familiar. That is normal and it will keep happening. The difference between people who make it and people who do not is the willingness to keep choosing differently, even when it is hard. You will fail sometimes. Get up and keep going.
Here is something I read when I need a reminder of who I am choosing to be. It is from a book by Og Mandino, and it helped me. I hope it helps you too.
I will persist until I succeed. I was not delivered unto this world in defeat, nor does failure course in my veins. I am not a sheep waiting to be prodded by my shepherd. I am a lion and I refuse to talk, to walk, to sleep with the sheep. The slaughterhouse of failure is not my destiny. I will persist until I succeed.
You are 15. You have time to build something extraordinary. Start now.