PubMed
Many colleges subscribe to different interfaces of PubMed, but the free online version maintained by the U.S. government is actually quite good. It allows you to search using the MeSH vocabulary, narrow your search to just clinical trials (for example), and limit your search to articles freely available online (in what they call "PubMed Central").
To access PubMed, click on the link above (in blue), or, go to this URL:
Getting Started: Instructional Materials
The factsheet and videos above should provide you with enough instructions/examples to start conducting searches in PubMed. If you're interested in going a little deeper into what's possible in PubMed, however, we recommend taking 30 minutes to work your way through the training course below. Additionally, more material can be found at the National Library of Medicine's "PubMed Online Training" page.
Training Course: PubMed for Nurses