Actually peoples start with easy customer map like ( Paintball, Chop, Kotr, or BGH )
Paintball is a little bit retard from my point of view but... Kinda like a free units map where you one shot one kill, not really fun but anyway.
Chop is a good way to understand how peons are important into this game, by learning chop your gonna learn the basic for your future in pro map.
BGH is the same as any rts game, BIG GAME Hunter, where everything is large and free, a good way to learn your building and hotkeys but a bad way to prepare yourself for expensive pro map.
KOTR is almost the same as bgh in smaller version.
My advice for you is to play Chop or join GOW game for newbs, not for pro.