Development can be done on whatever system you want as long as it has a text editor of some description, although compiling needs to be done on a linux like system. Your best bet is either native linux, or on windows a linux VM. You can also use cygwin if you want.
You can checkout the source from svn://svn.rockbox.org/rockbox/trunk
If you wish to participate, then take some time to familiarise yourself with the project, have a look at the website, the manual and the source code, then sign up for the roxkbox-dev mailing list, and join us in #rockbox on Freenode.
Have fun!
P.S. Please use real words such as "someone" and not "some1".