The key to figuring out the appropriate level of evidence for a given study is to examine its methods and assess its design. Below, we've linked to selections from a useful series of articles by Ranganathan and Aggarwal detailing different study design types (their methods, the forms of evidence they produce, etc.). Use these articles to help you understand the specialized language of study design.
You may also find the Centre for Evidence-Based Medicine's (CEBM) brief study design article helpful.
PubMed is an ideal place to begin your biomedical research. Any full text links that say "PubMed Central" or "PMC" will route you the free full-text content of an article. If a full text link routes you to a publisher page with a pay wall, return to the PubMed article information, look for the journal title, volume, and issue, and plug that information into Journal Finder to see if the Rice Library has a full text version.
Remember, if we don't have it, you can request an electronic copy via Interlibrary Loan!