Hello andreamilspaugh, welcome to LQ,
in Linux most programs including the OS itself are Opensource, this means you can look at the sources of almost every program.
Customization is also possible with any distribution. Which one you choose will depend on your knowledge. There are distributions which are known to be newbie- or userfriendly, for example Ubuntu or Mint. Others are said to be for more advanced users, like Arch, Gentoo and Slackware which are highly customizable.
Take a look here: http://www.zegeniestudios.net/ldc/