Not being funny, but you are quite heavy for your height.
You might be better off with the professional advice of a doctor, nutritionist or personal trainer. Going out and just busting it down the road on a bike might not be a good idea for someone that could have other health concerns.
FYI, your body is most efficient at burning calories (and body fat) when you maintain your heart rate in its aerobic zone for longer periods of time. You'll have to figure out what your aerobic zone is, and the length of time you can endure exercise in that zone may be minimal initially.
If you cycle too hard, you'll be in your anaerobic zone. This is not what you want for calorie burning.
If you think you're young/healthy enough and don't want to go the professional route, a heart rate monitor ($40 to $150) should be your first investment.
It's not about how fast you move the bike, it's about how fast your heart pumps. Sometimes, there is a close connection, but often there isn't.
Good luck.