It's important to keep track of your citations in order to create your bibliography as well as to keep a record of sources you still need to find. It's up to you how you want to manage your citations - whether a specific notebook, a file on your computer, or using a citation manger.
This is a great tutorial that you can take to better understand what plagiarism is and how to cite information correctly. It includes tutorials, videos, and quizzes to test yourself.
Lit Reviews and Annotated Bibliographies