Hi K! If you school subscribes to RefWorks, that will do it for you, but your school has to subscribe. I use a program you buy, called Endnote, but it's not cheap (usually about $100 if you buy at the student rate through your college bookstore)--I'm an English prof and published author, so it's clearly worth it for me, but it's not necessarily worth it for only undergrad papers. ProCite is similar, but also not cheap.
As a previous poster pointed out, Word 2007 or 2010 will do it for you under the "references" tab. If you're using books, go to worldcat.org, look up the book, then click on the "citation" link, and it will show you how to cite the book in a variety of formats (MLA, APA, etc.) Free online sites that will format your citations for you include http://www.calvin.edu/library/knightcite/ and easybib.com For both of these, you plug in the information, and it will format it for you. None of these will check your paper for you--you still need to know what you need to cite, and where from--but they all will format your citations for you in APA form. If you need help figuring out what to cite (to avoid plagiarism! Please don't plagiarize! I HATE having to flunk students for plagiarism!), google the website for The Everyday Writer and you'll find information and tutorials for learning what you need to cite, how to properly quote, paraphrase, etc.
Congrats on going back to school, and GL!