A common thing to do when you are just starting out is to map your house. it won't be a release worthy map, but youwill learn allot in the process.
One way I have always gotten ideas was to just look at the tuturials and try somthing new with every map. nuclear_research (http://www.snarkpit.com/maps.php?download=219) began as an exercise in the use of env_beam and ReNoesque hallways. Manthren started as an experiment in triangle terrain. my section in the pitcrew map2 is quite an exercise in the use of func_trains. I have many more maps I never posted anything about which were targeted on other more basic things.
Another great thing to try is rats maps. they are horrible for r_speeds so you can't put much detail in, but the inspiration is right in the room with you. I think I've made two or three in all.
[addsig]