I need to use peer-reviewed journal articles for paper on adolescent mental illnesses.
I suggest you look in PsycINFO one of the library databases for the articles you need. You can find this database alphabetically if you click on the "Databases" tab from the library's home page. Select the letter P and find PsycINFO in the alphabetical list.
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 Peer-reviewed box and choosing journal article under material type. 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.
I suggest you look at the research guide for Psychology to get additional help with your research.