Writing and referencing
In written work it is expected that you refer to the sources used in your writing. Good source use shows that you have an overview of the subject matter, and helps you to substantiate your arguments in a convincing way. It also makes it easy for the reader to find your sources and check facts. You should always cite the source when you use the words or ideas of others. Not citing correctly is the same as claiming that these are your own thoughts and ideas. It's plagiarism.
If your teacher does not tell you to use anything else, you should use the APA reference style for citing sources and making reference lists.