Outcast is fun.
I've never made a map myself, but I don't think the map itself requires too much programming, although scripting enemies doing things and cutscenes would probably require that.
You can get maps at:
The Massassi Temple
(make sure you select JK2 when looking at the mods)
I'm sure there are some others - pcgamemods.com used to have some, but they're mostly dead now - but those are the main sites to find mods and maps.
There's a small tutorial on mapmaking at Massasi here.
It has a link to JK2Radiant in the tutorial.
Also, you might want to check out this forum