hello
friend ,i am 25 my weight is 45 i wanna gain wait would you please suggest me . what to do.

Eat more calories.

If you wait, at 30 your metabolism is likely to slow down and you'll probably regret ever wishing you could gain more weight.

I presume you mean 45 kg which to folks in the US who still use the archaic system of English measure is 99lbs. How tall are you?

A simple way to add calories is to choose the higher fat version of whatever you're eating. 2% or whole milk instead of skim, add a slice of cheese or a sprinkling of nuts or cheese to salads. Most of us would like to feel that we need to gain weight instead of trying to lose it.

Also, this will sound counterintuitive, but weight training can help build muscle mass and put on a little weight if you are consuming enough calories to account for the additional exercise.

Play more video games.

(Finally, a health and fitness problem I'm an expert at!)

I concur with Judy on the weight training. That way the weight you can will be muscle and not fat. But you are going to have to eat a lot more than you are right now.

I concur with Eris