Source code to see how it should look like

Question

Does someone know how and where should i look or find some open source code?
As a beginner i belive i should see some larger piece of code so i can see a bigger picture of how to structure my future code (even if i did some little things, atm i have a program with an GUI and stuff with over 3000lines of code).
Maybe some code with multi-files(headers+cpps).
Probably i should see some complex structure, but an easy code so i can understand it as a first step, until i "build" my one style.
So,any links/ideas?

Answer 1

Sorry if i was ambiguous :)
@Martin Taylor Thank you for the answer! You belive i should search something in particular?