First off, dropping out when you have another choice is, in my eyes, a very bad idea. A high school diploma will take you one more year, and will benefit you for the rest of your life.

The most important thing is attitude: Believe in yourself. Believe that you can do this. Because if you don't, you'll just start undermining yourself after a setback. And you will have setbacks. Everyone does. The first step is caring about school.

The next most important thing is taking time to work on school work. You're behind, you will have to catch up. But if you invest work into that, that's achievable! If you have friends that are good in a subject, definitely ask them if they can tutor you - that'S a really good war for both of you to get better, and is much more fun than doing everything by yourself. On a more general note: Be aware how your group of friends influences your behaviour. They might start pushing you to do less work and take school less seriously - don't listen to them. Keep in mind what your goals are, and what's really important (which, cliché as it might sounds, is your education).

Another important factor is perseverance: don't give up, especially when it's hard. The harder a problem is, the more fun it is to solve! After a time you will notice that you will have an easier time at this, but the beginning is not so fun.

Never be afraid of asking questions. Yes, sometimes they will be stupid and sometimes you might get laughed at - that's okay. It's still better than not understanding something. Or if you are afraid of people's reaction, ask the teacher after the lesson. Also try to participate in the lessons - it's much easier to be immersed in the subject if you take an active interest.

Think about why you failed your classes, before. Can you tell us a bit about that?