Diet is a big factor, and sometimes the bigger factor than exercising. If you exercise consistently, but don't change your diet or keep track if it, chances are you won't lose weight.
A simple tool to figure out how many calories to lose weight
(not entirely accurate as everyone's metabolism is different)
Sedentary - 10 x Body Weight (in lbs)
Moderately Active - 11 x Body Weight (in lbs)
Active - 12 x Body Weight (in lbs)
Very Active - 13 x Body Weight (in lbs)
To get an accurate result, find your local doctors office or gym that offers metabolic testing.
TIP: Keep track of how many calories you have eaten in a food journal, or sign up to calorie counting websites such as..
caloriecount.about.com
sparkpeople.com