Most people have heard the term aerobic. This basically means keeping an elevated heart rate for a certain period of time. If you want to get specific, take 220 minus you age and then stay between 60-85% of those numbers. If you want to make it easier, get a heart rate monitor or just walk, bike or step until you breath becomes slightly labored. If you can talk you're not working too hard.
The trick I've found if to do something that you like or at least don't hate. Walking seems to be a good fit for most beginners and ellipticals for those who want a little more of a challenge. The first thing to do is to start small with 10 or 15 min. sessions. Try two to three times a week and build your times up from there.
If you want to kill two birds as they say, try circuit training. This is what most of my clients prefer because you tone and burn fat at the same time. All you have to do is follow up a few exercises with a min or two of walking or stepping. You'll keep the heart rate up, tone your body, burn fat all at the same time. The best thing is how fast the time goes.