Everyone else here is more experienced with bikes and can give better advice, but as far as losing the weight, I will say modify your diet! Cut out sugar, sweets, soda, cakes, and cut down on carbs. Scientists are slowly but surely coming to the realization that sugar is what's causing our obesity problem in this country, and our foods are loaded with it.
I was in your place, getting close to 300. After reading some articles online, I cut down on my sugar intake and instantly started dropping weight, and while my weight has plateaued at around 245, if I were more strict with myself on the amount of carbs I eat I could lose even more.
This is a very important step, because no amount of riding is going to help you lose weight if you don't first modify your diet. There's a saying that you can't outride the fork, which means you can ride all day long but if you don't change your eating habits it won't make much difference.
And I agree with others that you may want to get checked out with a doctor before you start riding just to make sure everything's OK.