Online Citation Tools
All of these sites allow you to type in your citation information (author, title, etc.) and choose the citation format you want (MLA, APA, etc.) and they will create a citation in the correct format which you can then copy and paste into your document. Always be sure to review the citations before you turn in your paper, though, to make sure they are correct!
Purdue University's Style Guides
Purdue University's Writing Center has created excellent style guides for the most popular citation styles.