How long does it take to get some kind of residency in the USA?

My girlfriend is coming to live here in the States with me by the middle of July, I was wondering what it takes to get some kind of established residency in the USA. I dont want here to stay here as an Illegal for too long, but i dont want her to be denied and deported. I was looking at such things as a student Visa, because she does need to get her GED. Any help or suggestions?