First of all, I'd recommend you be careful with hooks. Because if you make a mistake more than your program may crash, though if you're only going to hook a single notepad process there's not much of a risk.

Yes, you need to have the main program load the DLL to inject it into the notepad process. The DLL would then do whatever you want it to do from inside that process.

I'd very much recommend reading the MSDN documentation on hooks. There's a decent amount of sample code and explanations of what needs to be done.