I need to use peer-reviewed journal articles for a paper on adolescent mental illnesses.
Answer
I suggest you look in PsycINFO one of the library databases for the articles you need. You Go to the library's homepage>> the Databases tab>>"P">>PsycINFO.
You can begin your search with the term "Adolescent*" and the phrase "Mental illness" in separate boxes in the initial search screen. You can limit the results on this screen by choosing to check the Scholarly (peer-reviewed) Journals box and choosing journal article under SourceType. You may want to apply a limit to the time of publication if you need only recent articles.
I did a similar search and the result was nearly 500 articles. The sort of the material is by relevance. You may want to adjust the time of publication to get more recent articles or select a particular limit like "empirical study" to focus you results on exactly what you are looking for.