Have a look at
my site, it's got a page about Linux tools. It will get you up and running with the tools you need for compiling programs from the command line.
As for graphical IDEs, as with everything about Linux you have a choice
. For KDE, there is
Kdevelop which is what I use. It's very good as a general editing/building/project management environment. It can also become a full debugging environment, I'm looking into how to do that with
mithris' debugging stub so you can debug your program while it is running on the GP32.
There is also
Anjuta which is a very polished IDE, and several others that I haven't bothered to look at.
As always, the first thing is to learn to program on your PC, before moving to the GP32. Get comfortable with making programs that run on Linux (print "Hello, World", print all numbers from 1 to 100, open/read/write/close files etc). There are heaps of resources on the internet for learning C.
By the way, learn C rather than C++. If you need C++ then use it, but C is simpler for starting out with.
And good luck