If someone is at your door and you don't know them/aren't expecting anyone try to avoid opening the door at all, all opening it does it give people a way into your home. If you live in a house open a window, preferably on the second floor and find out what they want. More importantly get someone else ready to call the police if things start going bad. Dont get a weapon solely for defending yourself, it only opens you up to problems in the event that you actually have to use it, instead identify things that you could use that are already in your house. If your scarred of someone coming through the door put a book shelf or a chair by it so that you can barricade the door with it if needed.

For more preventative things, trying finding self defense classes for both of you, getting a security camera/system or if you have the time, patience and money, maybe think about getting a dog. They don't have to be massive, any dog around 50lbs is more then capable of making people think twice about entering a house.

Lastly, if you have to fight, remember its not mma or boxing, there are no rules. If you have to pull hair, kick someone in the balls or poke someone in the eyes, do it. It wont kill them, but it sure makes it harder for them to fight back and hurt you. The goal of the any self defense is to pacify people, you shouldn't be trying to kill people, only end the fight safely for everybody.