Running with quite a bit of extra weight, where do I start?

How overweight are we talking, and how long has it been since you've run? The most important thing is to start low - brisk walking or walk-jog intervals in short bursts to avoid putting too much strain on your knees, and maybe do it 2-3 times a week so your body can adapt without getting injured.

Don't just go out running without patience, because you'll end up injured and frustrated. What works is alternating: walk for 2-3 minutes, jog easy for 1 minute, repeat. Do it 3 times a week max at first, take rest days in between, and when you feel your body can handle more, extend the jogging intervals. The important thing is consistency over the long run, not burning out in the first week.