I want the player to press a button (probably tab) and bring up an in-game notepad that they can write on. I don’t have much experience in C# and I don’t know where to start
When you say “write” do you mean ‘type words’ or ‘draw words’?
This is a kind of advanced thing that covers a few coding topics, such as UI, user input, and also save game stuff if you want these notes to persist between sessions.
If you don’t have handle on these subjects, then you should learn them beforehand. Then you can use those skills to start tacking this problem with more of an idea of what to do.