I am 48 kg and 5 ft 8in tall. i've joined fitness center and i am working hard on it. it has been 3 weeks but i've not found any gain in my weight. ?

Food plan?: After your workouts, you'll need extra protein. I don't know what your usual diet is, but this is extremely important if you're trying to build muscle. You can work with a nutritionist and trainer -- together they should be able to help you create a program that moves you towards your goals. 3 weeks is not really long enough to see a difference, though.

Increased calories: Is the primary requirement here. First, it's essential to calculate your resting and exercise bmr, track your intake over 2-3 days, and write down the difference between the calories consumed and the calories burned. Take the time to do this; it's not rocket science. Once you can actually see that you're not eating enough quality food, you can take action! good luck!