Put together a list of relevant sources for what you are researching and writing, and analyze those sources to make sure that they are credible. For example, when using Internet sources, try to use .gov or .edu websites, as the information on such sites is usually credible and unbiased. If you aren't sure whether or not a source is relevant to your research or should be trusted as credible, ask a school librarian or your teacher to help you review the source.
Make sure you know what citation style you are supposed to use when writing a paper with sources so that you cite your sources and format your paper appropriately. Different disciplines use different citations styles; for example, humanities fields will use Modern Language Association Style, while social sciences and some sciences will use American Psychological Association Style. If you aren't sure what citation style your teacher wants you to use on your assignment, make sure to ask. Each citation style publishes an official handbook that outlines how to cite sources within a paper, how to cite sources at the end of the paper and how to format other aspects of your writing. If you don't own the appropriate handbook, check with your school library for copies.
Once you know what citation style you should be using, make sure that you appropriately acknowledge all ideas and wording in your paper that comes from outside sources. If you use exact words, you need to put quotation marks around the exact wording and include a parenthetical reference to your source at the end of the wording (the formatting of your parenthetical reference will depend upon what citation style you are using). If you are using an idea from a source but phrasing that idea in your own words, be sure to include a parenthetical reference at the end of the sentence where that idea concludes (again, the formatting of your parenthetical reference will depend upon what citation style you are using). If you are unsure as to whether or not you should cite something in your paper, show a draft to your teacher and ask for assistance.
If you write your paper at the last minute, you are more likely to unintentionally plagiarize by neglecting to closely read a source, forgetting to cite a source or citing a source incorrectly. Writing a draft of a research-based paper ahead of time gives you the opportunity to ask your teacher or a tutor to review your writing and your use of sources so that you can ask questions about any citations or formatting issues. A teacher or a tutor can also alert you to citations you may have left out.