The first step is the most difficult to take. Figure out logistics: he won't leave your shared home, so you'll need to find somewhere to stay..do you have savings, are you employed, can you find another place to live? Gather up important documents you have around the house, your passport, social security card, car title, anything official with your name on it. You don't want to be looking for that stuff when you're trying to move quickly, and he'll be super unpleasant once you tell him you're leaving. When you're ready to tell him, tell him. It's not a debate or discussion, you don't want an argument, no need for name calling or a big production. "I'm leaving you. I'll be moving out this date."