I want to develop a password manager, something I can run on my PC but mostly, I want to run it on a separate thing, like an RPi, but that’s an later thing. How can I plan this? I’m not a developer, I’m new to programming,
I’m learning C in school, is C good for this project? Any tips would be helpful since I don’t know where to even begin… 
C is a great language. But. I highly recommend people not write new projects in C unless they want to practice C so they can maintain old C code.
I recommend newer alternatives, such as Rust or Go over C/C++.
In C you’ll largely have to scratch-build large portions of your application and debugging is harder, while in Rust or Go you have better access to libraries and better debugging.
While I agree with Jeremy in general , if you are currently learning C in school then I think it would be an awesome idea to build a first version of this in C. It will help you understand the language so much better to have built something you want to have work.
And maybe if you’re taking another language next semester you’ll see an opportunity to make a better version of your tool in that language.
I say get started on making the thing, even if it’s not very good yet. If all you know about C so far is reading and writing, then make a password manager that just writes to a text file. It’s not secure, but it’s a first step. Have fun with it!
Happy Coding!