This is an essay which by nature will require expert/scholarly sources that are current (10-15 years old), because of the ever-changing nature of medicine and availability of both prescription and over the counter drugs.
While I cannot be considered an expert on this subject, I can at least point you in a direction to help you get starting finding some materials that should be helpful. "Google Scholar" is a relatively new (and free) resource for researching journal articles on just about any topic. Many articles are even available in full-text to read at no cost.
The first hurdle you must overcome is the "keyword search." It is always difficult to begin research on a subject you know very little about. Start with a few obvious keywords, then, as you find potentially useful information, search for other keywords from these articles to search further.
With your topic, I'd probably start with keywords [abuse] [and] [over the counter drugs] [teenagers].
Likely, this will produce something useful. Browse through a couple articles and see if there is one or two particular drugs that seem to be abused more than others. Let's use "Tylenol PM" for example. You could then do a search for [Tylenol PM] [and] [abuse] [and] [teenagers]. If you are not finding anything, leave out the [and]'s and see what happens.