How Do I Quit Alcohol and Drug Addiction?

My future is at stake here. I am a college junior majoring in biochemistry with a 4.0 GPA… this semester I got in deep with drugs and alcohol and now my grades are beginning to suffer… drugs and alcohol have taken over my life. I so addicted. I am losing friends left and right and my professors are wondering what the hell is happening to me. I am afraid if I don’t change my ways soon, not only will my GPA fall, but I might also die. My addictions are scaring the heck out of me. please help. Any advice is greatly appreciated.