For the people playing rated it follows that the map selection is random. In theory, random map means just that, they are suppose to be random. If you're just starting out, you may want to play advanced set up and look for land maps- they are a bit easier to learn on. Many new people like to play Jotunheim since it is easy to defend, or River Nile is nice because it cuts down on early rushing. But eventually people should play water maps as well since if you aspire to play rated quick setup, RM is the norm. Keep an eye out for some map guides that are coming out soon or check the one on Tundra at the link below- it may be of some help:
http://aom.heavengames.com/cgi-bin/forums/display.cgi?action=ct&f=1,17755,,all