Zotero is a browser add-on that collects, manages, and cites research sources. It's easy to use, lives in your web browser where you do your work, and best of all it's free. Zotero allows you to attach PDFs, notes and images to your citations, organize them into collections for different projects, and create bibliographies.
Check out this Zotero tutorial video from Ohio University libraries
Purdue University has an excellent online writing lab (Purdue OWL) that assists students with academic writing. Purdue OWL has citation guides at: https://owl.english.purdue.edu/owl/section/2/