Use this reference for Zotero: http://guides.library.oregonstate.edu/zotero
Zotero is a free, open-source research tool that allows you to manage citations and build a searchable online research library. Using Zotero, you can annotate and organize your research documents. Zotero integrates with word processors, including Microsoft Word, so you can cite-as-you go. I know there is also a way to use it with RStudio, but I haven’t actually done that.
Go to the Zotero download page.
If you use Firefox, you will want to download the “Zotero for Firefox” version. Otherwise:
I highly recommend getting a Zotero account. The account is free, and it allows you to form groups with other people and access your citations from anywhere!
If you want storage space so that the files (pdfs, screenshots, etc.) are stored with your citation metadata you must purchase storage:
https://www.zotero.org/settings/purchasestorage
If you think you will be using Zotero, I think this is worth it!