If your program has a capstone project, you may need a test or measure to use in your project. This page covers:
To find instruments relevant to your research project, look at articles on similar topics in peer-reviewed journals. The Methods section of those articles will describe how they measured the constructs you're interested in. This will help you in two ways: you will understand the entire topic more deeply, and also you will get ideas for instruments you could possibly use yourself. For example, if you're studying the topic of cognitive functioning, you would read articles and note which cognitive inventories were used and why.
After you do the previous steps, you should have a sense of what tests have been used to study your topic. If you are stuck here, contact your professor or your librarian. Otherwise, you are ready to evaluate whether the tests are valid/reliable enough for your topic and whether you have access to the tests. The databases PsycTests and Mental Measurements Yearbook are very helpful for this.
You will need to show that the instrument you've chosen is valid and reliable. There are scholarly studies devoted to assessing the validity and reliability of instruments. To find these:
If your instrument is related to mental health, check PsycTests or Mental Measurements Yearbook. They compile reviews of many psychological assessments.