For example, if major journals for my identified topic were "Rehabilitation Counseling" and "Journal of Rehabilitation Research and Development" the major databases I would search would be: MedLINE and Scopus because they index these journals.
Check with your professor but typically 3 major databases is sufficient in conjunction with searching conference proceedings, clinical trials and hand-searching when necessary for your topic.
Need the PDF/full-text?
Use Article Linker to search for full-text across all databases. If the article isn't available, choose Request via ILLiad or use interlibrary services to request a copy from another library.
Using Google Scholar?
Sync your Google account with USU. This will get you past paywalls for articles USU subscribes to by having you login with your A# and password.