Whether you use note cards, a word processing program, a database or a web log, keep careful track of your sources. You will need that information later to do your bibliography.
Be consistent in your note-taking. Try to keep all your notes together in one place. This will save you time and confusion later on when you're ready to write.
Make sure you indicate in your notes when you have copied something down word-for-word and when you have summarized or paraphrased. Remember that a quote has to be indicated as such by quotation marks, and even a paraphrase should be cited correctly.
What's a paraphrase, you say? Read on!