Once you have your topic, before doing any searches in Catalog or article databases, please analyze your topic.
Please remember, you cannot write everything about depression or abnomal behavior -- try to focus on some specific topics, such as depression with a specific group of people, or specific treatment for depression, etc.
To analyze your topic, you need to draw the key concepts from your topic. See this example.
Topic: Is play therapy helpful for children with separation distress?
The concepts drawn from this topic are: play therapy and separation distress
I encourage you to analyze your topic first, because you may not be lucky to find a book or article use the exact sentence as your topic. But once you know what concepts you are looking for, it will be very easy to search the concepts as keywords when you use the library catalog or article databases to find what you want.