and how do i know, i did not start to burn muscle weight instead of fat ?
In other words: how to prevent losing muscle while burning fat exercises?
to prevent losing muscle mass you have to do some form of resistance training (i.e. weight lifting). you should make a workout routine which targets all the major muscle groups.
heres a site which has some good routines, if your not an experienced lifter
your diet is very important as well, id say its more important than the exercise itself. you'll want to cut out all the fatty foods from your diet, things like crisps, chocolate, chips etc. a great way of cutting down unneeded calories is to replace all your drinks with water. water has no calories at all, and its exactly what the body needs for hydration.
the best foods to eat are rice, oatmeal, vegetables, fruit, and lean meats like chicken breasts and tuna.
metabolism is important in breaking down fats. in order to speed up your metabolism you'll want to eat 5-6 meals a day. i know 5-6 meals sounds like alot, but they should be small meals, over 3 big meals a day.
don't expect to lose the weight instantly either. you may not notice much difference at first, but just keep at it and within a couple of months you'll be a thinner healthier you!