You'll want to start out a little faster than what is necessary to get your target time. Psychologically, you're going to slow down as you get further into the mile, so make sure this first lap is good to compensate. At the same time, don't spend all of your energy on this lap. A good example would be, if you want to run a 5:00 mile, each lap would need to be 75 seconds. A good time for your first lap would be 71 to 73 seconds. It's not too fast, but it's fast enough to give you some security.