Hi Nelnel! Kudos to you for understanding how important it is to lose FAT and not just lose weight!
Preserving our muscle is soooo important for keeping our metabolisms running high and for looking tight and fit when we get to goal.
The best way to make sure that you're losing pounds of fat and not muscle is to weight train while you're losing weight. Your goal is to maintain the lean body mass that you have and maybe even add to it, if possible. And that means lifting weights!
Your weight training should be accompanied by clean eating and enough protein to help build and repair muscle. You want to make every calorie that you're eating as nutritionally dense as possible - really get a lot of nutrition 'bang for the buck' out of every calorie. I don't think that low calories by themselves will cause you to lose muscle, but low calories + poor nutrition and/or no exercise to maintain muscle mass WILL result in muscle loss.
As you're losing weight, it's a good idea to keep track of your body fat percentage so you can tell what you're losing: pounds of fat or pounds of muscle. It will probably be some of both, but the goal is to minimize the muscle loss as much as possible. Have you ever had your body fat % checked? Do you belong to a gym? Have a Tanita scale?
Best of luck to you -- I'm so impressed that you're aware of the importance of muscle to your long term success!