building ubuntu-based distro?

[David Feldman]

I'm interested in playing around a bit with the Linux (probably GNOME) 
UI, and ultimately creating a distro with an ISO installer that I can 
try out on my friends. It will involve changing some defaults, 
including and excluding specific apps, and perhaps modifying some of 
the UI code. I think Ubuntu might be a good starting point. While I 
have a decent amount of development expertise I've never really done 
anything like this before, and in the interest of time I'm hoping to 
avoid modifying code unless I actively want to make changes to its 
behavior.

How would I learn how to do this? Where would I get the necessary 
materials?

Thanks and sorry if this is OT,
--Dave