There are generally 5 levels of doneness for a steak (this is what a server will ask you about if you order one at a restaurant). Here's a guide on how they look, and the temperatures they should be. The steak will continue cooking while it rests, so this also give you an idea of when to take it off the heat. This way, you get the exact end result you want after the steak has finished cooking. Follow the link for a full guide on how to cook the perfect steak.