As Blanky said, I think you are wanting to do mapping rather than modding. Although mapping comes under modding, modding is usually for actually changing the code(programming) to do something else.
All equally fun in my opinion.
Check out
www.qeradiant.com for GtkRadiant 1.5 and search the forum on how to setup GtkRadiant to work with OpenArena. After that go and read up on the tutorials on how to make maps.
Heres a link to some basic/advanced mapping techniques you can use. I suggest you look at basic level design first.
http://bubba.planetquake.gamespy.com/mapping.htmlIf you have any troubles getting the map to run in OA or compiling the map just ask.