Obviously, the simple answer is: what does your teacher say? Depending on the assignment, that answer may vary.
In general, however, there are two, more complex issues to consider.
- If you are using someone else’s work in your project, you need to give that person or organization credit for their work by citing your source. There is lots of information in the Writing Manual about how to do this correctly. But, more importantly: you can make your work stronger by basing it on strong, credible work from other authors… and if you don’t cite that work, no one knows about that connection and either a) it does you no benefit or b) worse still, you are plagiarizing their work (that is: presenting someone else’s work as your own), which is a major moral and ethical problem (not to mention violating a major school rule).
- Some things that you can download from the internet can be legally re-used. And some can’t. And some kinds of re-use are legal, and some aren’t (even if you cite them completely correctly and properly and do everything with the best of intentions). Different types of copyright and licensing apply to different types of media. It’s smart to work through a tool like the New Media Rights App to decide if you can re-use someone else’s work legally.