I am 22 years old.but still 41 kgs only.how can I be fat.plz help me.

You do NOT want to be fat. You want to make muscle with weight training and yoga.

How can I make muscle with weight training? I have no idea.I am an undergraduate student of software engineering.so I have to study hard.but with that I want to make me better with my appearance.help me.

Google keywords like weight training exercises or yoga postures or whatever combination of words floats your boat, then choose maybe six and do them in sequence twice a day for a half hour each time.

I will do it.how long will it take to make my muscles by doing yoga?what kind of food should I eat during this?

Eat a balanced diet and avoid sugary snacks and drinks. Fruits, vegetables, lean meats, grains and nuts, lots and lots of water.

Weight yourself now. Do this program for two months, then check your physique and weight again. This will improve posture and strength and will smooth your body -- especially the yoga will be good if you do the postures correctly.