Most cases of plagiarism happen because the writer doesn't know how to give credit to the authors she draws from. If you know how to give credit to an author, you don't have to worry about unintentionally plagiarizing from your sources.
Take careful notes, especially when you're working with Internet sources, and always record the author's name next to any notes on his text. If your notes are sloppy, it's easy to forget where you found the ideas, and you might even think they're your own ideas. This can result in your final paper containing some plagiarism.
Place any words from a source in quotation marks, whether you're recording the words in your notes or in your finished piece of writing. Especially in this age of cutting and pasting from electronic texts, you can overlook placing passages in quotes, leading to plagiarism.
Paraphrase and summarize sources accurately. Make sure your paraphrases and summaries are entirely in your own words and sentence structure to avoid plagiarism.
Introduce authors in the body of your essay, using attributive phrases wherever appropriate. An attributive phrase makes it easy to give credit to authors and also makes it easier to refer back to their ideas. For example, including a brief phrase before a quote, such as "According to Bessie Smith," tells your reader who said the following, clearly giving credit.
Discuss authors and their ideas in the body of your essay rather than viewing their ideas as absolutely authoritative. When you engage in a discussion, you can show where your authors' ideas end and yours begin, which then allows you to credit yourself for ideas while also giving the authors credit for theirs.
Use the correct format for whatever citation style your instructor has asked you to use. Each citation style has a slightly different format to emphasize whatever information is important to the field that uses that style. Using correct format is an important aspect of giving credit because it allows your readers to easily locate the sources you used.