Although this is often easier said than done, by starting early you give yourself enough time to do a thorough literature search and document the resources you use.
DOCUMENT THE RESOURCES YOU HAVE USED
Keep track of the resources you use, documenting the information you will need to cite them. This is important whether you are taking exact quotes from sources or you are gathering ideas to use or paraphrase. The minimum information you should record includes:
the author's or editor's name
the title of the work
the publisher and date of publication
for web sites, the URL and the date you accessed the information, especially if some of the above information is lacking
You should also keep this information until your paper or project has been graded and returned to you.
LEARN HOW TO CITE
There are many different styles of citing resources, many of which are specific to a particular subject area or academic discipline. Be sure to check with your instructor about which style to use as your start your research. Nimitz Library's Citation/Style Guides provides some tips and links to resources to help you with citing.
Avoiding Inadvertent Plagiarism
The Harvard University Graduate School of Education has prepared an online tutorial titled "Principles of Paraphrasing" to help students avoid inadvertent plagiarism.