PubMed will show search strings similar to your research topic. Try using Boolean Operators (AND, OR, NOT) to separate keywords in your search. For this example, I want to find research articles on the effects exercise has on mental health, specifically depression. So I enter in the following search string into the PubMed search box: exercise AND depression
The simple search of "exercise AND depression" resulted in over 28,000 articles. To reduce the number of non-relevant articles you have to look at, try limiting the search using filters, such as DATE or ARTICLE TYPE.
To access the full text of an article, look for the Find It! button. This button will redirect you to the library's website where you can look at and download the article for free.