Give credit where it is due, it is the smart (and ethical) thing to do.
Bonus! You also avoid plagiarism (theft of credit) by citing sources!
Citing a resource properly should allow another person to RE-search out the item and find the exact same content that you used.
In more detail:
Use only one citation style per project (APA or MLA most common). Ask your teacher what style they prefer.
Use cite suggestions provided by databases when available.
Ask your teacher if the database cite suggestion is good enough, or whether you need to further check your style with a style manual (especially in technical and upper level writing courses).
Copy/paste and alphabetize citations yourself, or use a citation manager such as KnightCite if you have a large number of resources.
KnightCite creates accurate and updated citations. You get a free account to store all of your citations so you can keep track of all of your resources for multiple classes. You can export the alphabetized and formatted citations.