MLA / APA Citation
These are online resources that "build" citations for you when you input information. You should always check the final citation against recent standards and what you've been shown in class.
Keep in mind that the purpose of citation is to give credit where credit is due, and to lead your audience to your original source.
This project from Carnegie-Mellon makes citation shamefully easy...meaning you would be ashamed if you don't get it right.
 It's easy...
Citation Machine
 The Landmark Project
 This generates citations for books if you have the ISBN
Wikipedia citation generator
 If your teacher lets you use Wikipedia they have their own citation generator

 Here are some more links with writing and citation information: