You pay for this center. It can be done inadvertently, usually by taking notes directly from something you are reading. When in doubt, footnote all your references and put direct citations, even short phrases, in quotation marks. You do not need any outside sources for most essays. You are welcome to use them, however, IF you cite them.
Introductions are important.
Conclusions should pull together the different points you are making. But don't just restate your introduction. This is a good time to reread the question and make sure you have answered it. If there's some doubt about where your arguments have been going, use the concluding paragraph to reconnect with the question. Be sure to use words correctly.