There are several terrain making tuts right here on Checkpointdelta go to the terrain dnload section goto the bottom it will say tutorials.Dnload all of them and read them,just to get a feel for what you need to do to make a terrain.Everyone is differant, one of the tuts may help you more than another.
My first terrain was made after reading Smokins tut so thats the one I'd suggest to read first.
You do need to pracitce with the paint program of your choice for a little bit before any decent terrain can be made.