Good luck at the gym tomorrow. I agree with the other posts - start slow and work your way up to more intense and longer durations. If you get too sore and/or injure yourself it will take a lot longer to lose that weight.
My trainer always reminded me that food is 80% and working out is 20%. Ensure that you are getting enough calories to accomodate your workouts and to keep your metabolism going. Common rule of thumb is cut 3700 calories a week to lose one pound of fat. There are a lot of websites that can help you find what your calorie target is and you will adjust to your individual needs after that.
One more piece of advice, after awhile, incorporate weights into your workout - it burns calories for a LONG time after and will help tone you up. And if you are unfamilar with it, an investment in a good trainer is worth every cent!
Have a good day the gym!