A citation manager will make keeping track of your sources and formatting your citations much easier, and it's best to set one up before you start collecting them.
I recommend (and provide support for) Zotero, which is free and runs on Mac, Windows, Linux, and iOS. Zotero integrates with your browser to save citations to the sources you find, along with full-text PDFs when they're available. You can add notes, annotate PDF files, and generate citations within the app, and you can use Zotero integrations for Google Docs and Microsoft Word to easily add citations as you write.