What category of app are you looking to write, narrowing down the class "app with a GUI" a little?

How are you normally expected to compile open-source programs?

Can you name a specific example of one you've tried to compile and run, and you've been confused at the result?

One general hint is that a good way to learn how to code up significant programs from scratch is to, first, get a significant program that works and modify or extend it in some way.

Also, be aware that there are several competing design philosophies when it comes to writing GUI programs, with very different outcomes in terms of maintainability and adherence to sound design principles. The "Visual" approach exemplified by the Microsoft line of tools leaves much to be desired in my experience, leading to spaghetti code too easily.

I prefer approaches in which graphical components are created programmatically, and where design principles such as MVC then serve to further structure the resulting code and drive the design toward high levels of abstraction. The various Smalltalk environments are a good illustration of that philosophy.