Keep track of your sources.
Start saving your sources as you find them. Save them to a citation management tool (like EasyBib or Zotero), email them to yourself, or print them out.
Take good notes.
Keep track of page numbers and mark the sections you are using. Keep a research log; copy the wording, punctuation, and spelling exactly as it appears in the original text.
According to Plagiarism.org, re-tweeting without giving the original source is the 10th most common type of plagiarism.