Citing original research articles (not encyclopedia) is critical for your formal report
Most of your references will go in your Introduction and Methods, with a few elsewhere
If a wikipedia (or other website) has a particularly good article that helped you, you should also cite it (cite the "permalink" for wikipedia).
Example: Speed of light
Citations within wikipedia not great
When you find good references, add them to wikipedia!
NIST CODATA is a great resource for us