Quiet_girl, just start with something extremely simple. Dare I say it...a killbox. I wouldn't even worry about making a DM map at first. Just make a SP map with a info_player_start, a light entity with default setting, a rectangular box surrounding everything, and perhaps a weapon.
Once you got the hang of compiling your map (and make sure that you turn RAD on when you compile), then I would proceed to more complex things. Just use one texture for all the surfaces at first. Worry about beautifying your map later.
My first map...you wouldn't want to look at it. It might cause your retina to detach.