Hi. First of all, are you positive that you need to lose weight? Be realistic. Not everyone looks like the super skinny fashion models and, in my opinion, that is neither healthy or attractive.
Check out this site and see where you are healthwise and fitnesswise. It will tell you what your BMI is, your target heartrate, body shape (how you store your fat) and many other useful things, including whether you are overweight. I like it because it asks for your age.
http://www.ivillage.com/diet/tools/healthcalc/
At your age you need more calories and fat than us old ladies. Approach this with caution. Stay healthy.
In the meantime, we are here to support you.