I'm 21 y/o & underweight, my instructor has told me to take a mass gainer. please suggest one that's cheap and good. thank u :)?

No such thing . . .: There is no such thing as a mass gainer, at least not in the States, although I can't speak for India, certainly not cheap & good. In other words, eat as much good & healthy food as possible, especially protein from various sources, both vegetable & animal. Protein supplements in form of powders to be mixed are available in States. Again, I can't speak for India & other parts of globe.