I weigh approx. 140lbs and I'm 5'7", female. If I go to the gym to use the ellilptical for 20 minutes on easy, and then go on the bike for 20 minutes on easy, 3 times a week, how much of a difference do you think I'll see in a month?

Also, this will be my first time going to them gym. Should I stretch before, during? Any tips??