My advice is never to use it. At the risk of offending someone, it is really only used by black people. It is based off the word holler. This means to call or yell. You can use holler but holla would sound rediculous coming from you. Holler isn't used exactly like holla. It is used more like yell.

http://www.urbandictionary.com/define.php?term=holla check the link