Citation styles can vary by course and subject. You may even find yourself using a couple different styles, depending on your classes. You'll want to double check your assignment for the correct citation style to use, and ask your professor if you're ever unsure.
So, why do you need to cite in the first place? The first reason you're most likely to hear is that it helps you avoid plagiarism, or using someone else's work as your own. That is an important reason, but it's not the only reason. Citations give your readers ways in which to track down your sources. It helps them check your work, but it also helps them uncover how you're interpreting the sources and how those writers are addressing your topic. And it helps you when other writers use citations correctly. You can use those citations to track down more sources for your research and see how the topic has evolved over time.
*A note about citation generators: Citation generators (found in most article databases and online library tools) are great resources and can help speed up your work. However, remember that they are tools to help you along, but you need to double check all of your citations for accuracy.