Fallout 3...........?

You get a house in Megaton by disarming the bomb so until you do that you won't have a house there. The only other way to disarm the bomb without leveling the demolitions skill is to find something that boosts it to the required amount. Or you can just blow up Megaton(bad Karma) You get clean water from the robot in your house in Megaton which you get by disarming the bomb. So basically explore a bit and level up that demolitions skill.