Thread: Which platform to use?

I'm about to choose where to program in C (or C11 preferably).

I don't have any chosen platform (OS,GUI/IDE) yet, but I need my programs to be very easily portable to other OS's (primarily Linux & Windows).

I worry the most about the compatibility for windows in different OS's. How portable are these libraries? Can I use the same source code for any OS?
I will be doing normal programs, like random numbers, internet-connecting, static graphics, logging in, save-to-file, etc.

The question is, where do I get the most from a chosen platform to do this?

Should I use a GUI-program like Visual Studio (I have an academic licence), an IDE like Eclipse or just a raw pad like QT?


PS. I am reasonably new to programming, so I haven't really developed any preferences yet, so shape me in the right way, please
PPS. I must program in C, not C++.