How do I get the name of a package, modify and install it?

view full story

http://askubuntu.com – I'm not very familiar with Ubuntu or Linux, but I'm a programmer, and some people told me that you can just go ahead and modify your system. So my question is, how do you go on about that? For instance, If I'm interested in modifying the behavior of the button or the desktop icons, or whatever it is: How do I get the source code? I guess I need the package name and download it. But how can I get the package name? Let's say I want the button package, where do I look to get the package name? is there a list on a website or a help file? Once I modify it, how can I replace the original with the (HowTos)