You're still young and can fix this, but you've really got to buckle down. Here's some free advice (feel free to ignore it of course):

Too many people go to school without clearly defined goals. Try and find an academic discipline that excites you. Believe me, I knos this is hard. Consider areas you never considered before.

It's easy to get short-sighted and become lazy. Don't allow yourself to do that. Continually remind yourself of why you're in school and what your goals are. Always keep the big picture in mind.

Don't associate with people who are a bad influence on you, no matter how close you are to them. They will drag you down. This may seem cold, but it's your life and you've got to do whatever it takes to succeed. Professors and textbooks; THOSE are your new best friends.

A lot of kids go to school to party. Accept the fact that you're not in school to party; you are there to learn, period.

Mos importantly, you need motivation and self-discipline. Unfortunately however, these can only come from within. Reach deep and you will make it through.