Personally, I'd think something like this would be ideal for a web application so even if you're out of town you can still access your diary. You can do CGI programming in C since you already know it, but if you're doing this project to learn you might want to look into PHP with a MySQL backend or something. I've never gotten into that stuff myself, but that's the feeling I get
Some GUI APIs out there, if you'd rather not do the web thing, are GTK+ and QT. GTK+ is more geared toward C and QT is more geared toward C++, from what I understand. Does that kind of get you started?