Zotero is our top-recommend free citation management software. Through a browser extension, it automatically senses content and allows you to add it to your personal library with a single click. The desktop software then allows you to organize, take notes, and export your citations. Through plugins for all common word processing software, it can auto-generate bibliographies (reference lists) and citations in any format. It has been reliably produced and supported by a non-profit foundation for many years, so it will not steal your personal data. Check out the quick start guide.
If all you need is a web page that can quickly generate a citation for you from a URL, ISBN, or DOI number, they also operate ZoteroBib, which is refreshingly free of ads.
A quick diagram of the main buttons in Zotero's control bar:
For a great tutorial, see the YouTube video below: