Elliptical machines are the best overall exercise IMO. They work upper and lower body, are easy to use, low impact, and burn a lot of calories. If you don't want to join a gym, try to lease one from a fitness store instead of just buying one, if it's a reasonable monthly cost. Stick it in front of where you usually watch tv, and exercise away. I really would recommend joining a gym but never for a long term (more than a year) and never with Bally Total Fitness (their memberships are actually three-year loans and they've ruined the credit of thousands) or anyone who wants your social.
BTW, if you want to see a secret about good abs, go to
http://www.fatlosstips.com. Best wishes on your fitness goals, and remember that it took a long time to gain the weight, so it'll take a long time to lose it too.