Go here first,
Read it from the first lesson to the last, and try the stuff there..
After you can do the basic stuff, go here...
There is a message post, 3rd or 4th from the top of the level editing page with all links to Quake 3 tutorials, lots of archetectural links as well. Its a thread by Astrocreep....
The way I see it, its just like building with Lego's. There is some tagging to do to make doors and whatnot, but its really pretty easy.
Rich's tutorials is a good walkthrough for the basics...you can download all you need from there.