Always check with your instructor for specifics on your assignment, but typically certain disciplines use a particular style:
MLA: English, Art History, Music, Religion, Language
APA: Nursing, Psychology, Education, Social Sciences
Chicago-Turabian: History
Paraphrasing is very useful for writing papers. It can help you develop a flow to your paper and avoid using a lot of quotes. You can also gain a better understanding of the passage when you put it in your own words. But remember: even if you paraphrase your source's material into your own words, you still need to cite your sources. Here are some resources to help you paraphrase effectively:
There are many tools that you can use to help you with your citation. Remember that these tools are only as good as the information you put in - they can't check for accuracy. Make sure you double check all your citations before submitting your assignment.