Step 1: Establish what (medication or drug) they have taken, when, and how much.
(The emergency services will want to know this information.)
Step 2: Call poison control. Call 911 if the victim becomes unconscious, have difficulty breathing, or if you think they are suicidal.
Step 3: Don't try to make the victim sick or give them any more medication or drink unless instructed to do so by poison control.
(By making them sick, you can cause further damage to their throat.)