Appreciate the sentiment of wanting to protect yourself and your loved ones, but if you do go down this route, make sure you do it right.

Know the laws, take a class on how to use your weapon, know when to draw one and when not to. A friend of mine likes Armed Citizens Legal Defense Network, which regularly shares interviews with lawyers and police. There's a lot of discussion about local laws, how different situations tend to look when played back to a jury, etc. (i.e. knives tend to take a surprising number of stabs to take down an assailant, most untrained individuals see 15 punctures and instantly jump to excessive use of force)

If you do decide on a gun, think about if you want a concealed carry license. Others here have talked about shotguns and rifles too. In your situation, you might want something your girlfriend can grab if you decide to leave it at home. (Or both carry)

Separate from a weapon, also see if you can do anything about your home security. If you can, also try talking to your landlord and seeing about installing a better lock / guards on your doors / windows so any punk can't kick your door down.

Advice out. Godspeed.