I got my guy slimmed down pretty easily just from taking away all the dry and feeding him exclusively a good grain free wet food. The website
sugarcatmom gave you has a good run down of what the calorie count was per can. The best rule of thumb is to try and cut down her calorie intake by about 20% - then after about a week, weigh her - if she looks to be losing too much too fast, increase the amount, or vice versa. You can pick up a baby scale and weigh her at home if you don't want to take her to the vet each time.
Oriijen is a good grain free dry food, but it does have a high fat content, and cats don't need to be on dry at all, so you can save yourself some stress by just ditching it altogether. Jake was only on dry beforehand, but he made the switch fairly easily - I know some have some problems, but if your little lady is already eating wet it shouldn't be much of a problem.
As for exercise, a little bit at a time (just like for humans) makes a difference. My guy got more exercise because I had brought a kitten home - just running away from her got his heart rate up
I have a thread with pictures to show his weight loss on here if you are interested. Good luck!