I'm currently doing my own diet, which is basically eating a lesser amount of food, and selecting healthier, low-fat foods. Ive gained around 15 kilograms since entering college (since 3 years) , all due to plenty eating out! I really have to shed off those kilograms, and would like to know a suitable rate to go by, cz i heard that if i lose weight too fast, i might get flab and stretch marks! I want to make sure that I'm acctually losing FAT and not muscle.

healthy amount is 1-2lbs a week - it means you are losing fat not muscle.
i hope you know that the only way to lose fat is by doing cardio 4-6 times a week for 30-50min combined with light weight training (which also will tighten your skin)
(never go lower than 1400cal a day)

losing 1-2 lbs a week is a healthy rate. unfortunatly, those stretch marks are formed as you gain the weight in the first place...

i use the bowflex machine and it works i started getting small stretch marks but since i've used the machine every other day for 20-30 min they went away and i also eat whatever i want and have still lost weight.