I started riding after I sprained my ankle & couldn't run. Got hooked.
Start off slow. Do short distances. Increase them every coupla rides as your fitness improves. Bring a water bottle. Stay hydrated.
And it would benefit you to get a bike mag subscription for a year. In that time, you'd read about all the benefits, trix, things, to do & general bike knowledge.
There's a lot of social activity, too. Bike club's cater to all levels. There's benefit rides, getaways like ski mountains in the summer(lotta fun), week long rides where you can camp/motel, your stuff gets shipped along. Most every state has one.
As far as how many calories you'll burn, chk this site. Takes into account your weight, too.
http://www.exrx.net/Calculators/Calories.html
3500 calories = 1 pound of fat.
Good luck! I'm going for a ride now, really!