Filters can be found to the left on your Search Results screen.
Filters should be applied at the end of your search. Otherwise you run the risk of them being applied to only part of your search rather than all of it.
Only a few filters are shown by default. To access more, click the "Show additional filters" or "Customize" links. Once you have selected the additional filters you want to use, they will appear underneath the already visible limits. Then, click to apply them to your search results.
Useful filters include:
Sex of study subjects
Age of study subjects
Article types such as review articles or randomized controlled trials