I'm a sedentary male. If I cut my calories by 600 below my daily needs & begin to lose weight weekly, wil it be mostly fat loss with some muscle loss?

Weight loss. programs are best carried out under the supervision of your doc. We need to know your present weight, lab work, etc. Exercise turns fat into muscle so that needs to be part of your overall program. It can be light dumb bells, brisk walking, etc. You can do it and you will feel better. See doc first. Peace and good health.
You need to give us . your height, weight to calculate your BMI, and your HgA1C to help you out. We don't have info on your daily calories needed. Sedentary can mean overweight or? If you cut calories too much you can lose too much muscle mass too severely to provide the proteins you need. You'll burn through the glycogen stores, b/f burning fat. Yoyo dieting isn't ideal either. Best to talk w/ your Dr.& work together.
Diet. losing weight and less calories is good . If u do weight training u will not lose muscle.