Improve your note-taking skills. Each time you jot down a compelling fact, legibly document the source directly below so you don't forget later. Write down all of the bibliographic information you'll need when you type up your paper: the name of the author, the title of the study or book, publication dates, URLs if you use an online source. Whenever you pull a direct quote from a reference, enclose the sentence in big, bold, very obvious quotation marks, or draw a big "Q" next to the quote so you don't forget.
Whenever you're taking notes on information you plan to include in your paper but don't intend to quote directly -- close the book. Rephrase the information in your own words, without looking back in the book. Then, when you sit down to type your paper, rephrase the information again. Putting the original text through two separate iterations of your own rephrasing makes it much easier come to your own conclusions and to avoid "borrowing" your source's wording.
If a fact or statement is considered "common knowledge," it's not necessary to cite a reference for that fact or statement in your paper. However, sometimes it can be difficult to determine what "common knowledge" actually is. The phrase's somewhat nebulous definition is what lands a lot of well-meaning students into trouble. A good rule of thumb: if you find the fact in five different credible sources, it can likely be considered common knowledge. For example, you don't need to cite a reference when you state that the Battle of Bunker Hill took place June 17, 1775.
Keep in mind, a piece of information may be "common knowledge" to experts in the field, but that doesn't mean it's "common knowledge" to you. If you're a freshman taking Psychology 101, you may not want to wax eloquent on Lacanian theories in your paper without citing any references.
Ask yourself a couple questions when in doubt as to whether a piece of information is common knowledge. Assess if you already knew the information before you started researching your assignment. Alternatively, question if the statement in your paper is an opinion or an analysis rather than a concrete fact; determine if you came up with that opinion entirely on your own without any influence from outside sources.
You know you need to cite when you use a direct quote or include the results of a study you found in a scholarly journal. Many students are unclear, though, on documenting non-printed sources. So for the record, you need to give credit for words, ideas or data taken from all of the following: websites, TV programs, song lyrics, diagrams or other visual presentations, letters and emails, advertisements and interviews (whether you conducted the interview yourself or found it in any form of media).
If you're not sure whether a piece of information needs a documented reference, ask your professor. She would rather take the time to explain it to you than have to call your academic integrity into question. If you have a last-minute doubt and can't get in contact with your professor before the assignment due date, go ahead and cite. Better safe than sorry.
You've got the when, who and what to cite covered -- now you've got to figure out how to cite. There are a number of different styles for formatting papers and citations depending on academic discipline; this includes the Modern Language Association style for literature and American Psychological Association for social sciences, among others.
Usually, a course syllabus will detail the instructor's expectations for citing and formatting. Also check your school library for the handbook or manual for the style you need. Most styles require both in-text citations for quotations and a works cited, or bibliography, page. If you still feel unsure, just ask your professor.
Be aware that some students do deliberately plagiarize. If a classmate steals your work and attempts to pass it off as her own, you could be implicated in an academic dishonesty case. Password-protect any school related files on your computer, and set up a login password on the computer itself. If you work on a public computer in the library or computer lab, save your work to a USB flash drive or disk -- never directly to a public computer. Take the drive or disk with you.
If an unscrupulous student does manage to get a hold of one of your essays, you may find yourself in a situation where you must prove that the original work is yours. Save copies of your essays in different forms of media: one on your hard drive, one on a compact disk, and one as a printed paper copy. Instead of saving multiple drafts of an assignment into one file on your computer, use your word processor's "Save As" function to save each successive draft under a different name. Demonstrating your possession of earlier drafts can help you prove that you originally created the work.