How do the media and celebrities influence youths to do drugs?

Pretty simple: teens, or a substantial percentage of teenagers, witness the media output of celebrities on a day-to-day basis. The information these celebrities choose to feature in their artwork is processed by the teenagers, who may remember the works for a long time (especially if presented in song.) If the media features drugs, as it often times does, the teenagers remember and think about drugs. It's practically being advertised to them; displayed and glorified through these celebrities.
Chances are, if these teenagers are at all appealed by the celebrities, they will consider trying the drugs for themselves. The idea of doing something that a role model does is desirable and enchanting to the impressionable mind of a teenager, or any person who likes a certain celebrity.
Elementary, my dear Watson.