Learn about drug and alcohol abuse. You can even talk to the USI Counseling Center without giving your friend's name.
Prepare a list of specific problems that have occurred because of your friend's drinking or drug use. Keep these items as concrete as possible. "You're so antisocial when you drink" will not mean as much as, "When you were drunk, you made fun of me and were mean to me. You hurt me." Bring the list with you and keep the conversation focused.
Choose a private location where you can talk without embarrassment or interruption. Your friend is more likely to hear you in a quiet restaurant booth than at a large table in the UC. A talk in your room with the TV and music off will be more successful than one in your friend's room where s/he can easily create distractions while you talk.