Holla atcho girl is supposed to be used when wanting someone to holler at their girlfriend, which is a rarity.
Instead, most opt to use it when agreeing with something that has been suggested. It sounds like a cool response but really just means "I concur." If you don't want to sound like a pretentious Harvard student, opt for "holla atcho girl." Not intended for males.
"So I was thinking...we should go cliff diving this week. Whaddayasay?"