The website OWL from Purdue University is a great online resource for APA, MLA and Chicago style - both for citing and for formatting your whole paper.
NoodleTools Express is a great application to help you write citations. Chose the citation style, click on create citation and then choose type of resource you are trying to cite. A form will then appear for you to fill with the important parts of your citation. Click Save when you are done and you will get a citation you can copy and paste into your works cited page. Remember to double check all formatting and make your text is a uniform size and font (usually 12 pt Times New Roman) on the works cited page.