Start with the things that got you in trouble in the first place. Write some goals and a plan for how you are going to reach them.

Improve is another way of saying/asking you what kind of person do you want to become.
Define your goals, your values, morals and standards.
If it was peer pressure that got you into trouble, what caused you to give in?
Changing ones self is not an easy task to do, but raising your self esteem and by living by the rules you set for yourself will help you in many ways.

If you can choose a person you admire or look up to, how do they live there life? Can you learn from this person?

Choosing the people you hang out with says a lot about you, guilt by association.
I tend to hang out with people that will bring out the best in me and learn from them and how I can be more like them.
Dig deep into yourself and you will find the answers you are looking for.....

Good luck and stay out of trouble. ; )