When you research a topic for a paper or thesis, you will take notes on what you find. Make sure you thoroughly document the source of all your notes for later reference. Even if you don't take notes from materials you use, be sure to write down complete information on each resource you come across. You can decide later whether to include them in your paper.
Be sure to cite the source whenever you use another author's words or ideas. This can be done in the text, in a footnote or a combination of the two. You should be thoroughly familiar with the references format that is required by your professor or by the academic publication for which you are writing. Follow that format to the letter.
Create a "works cited" or bibliography at the end of your paper that provides complete details of every source you use in your work.
To ensure that you have complied with the spirit of the law, re-read your paper when you are done. Check whether the tone and style of your writing is consistent throughout. If you find that a couple of paragraphs here and there adopt a different tone or style, one likely cause is that you have adopted the style of your source. Consider reworking those passages, but do not hesitate to give credit for any ideas that you use.
If you still have concerns, check your work for plagiarism yourself using an online tool such as DupeFree or Copyscape to ensure you did not inadvertently plagiarize someone else's work.