See I am 18 years old male but I am so thin I mean I am thin . My details are as - weight 45kg , waist - 25 or 26(how can I increase it as it is too thin) height - 5.6 , how can I increase weight and waist I want to change myself I want best answer

You have a higher than average metabolism. You can take one of three approaches I will explain each one and what it does.
1) You can increase the about of sugar that you eat, especially sodas. Sucrose and Fructose mostly get converted into fat, and the also inhibit a signal to your brain saying that you are full, so you eat more than you would otherwise.
2) Crash diet, and then go off the diet. Crash dieting slows down your metabolism. This is why people reach a "plateau weight" when they diet. After your metabolism slows, it will be easier for you to gain weight. If you go this route, be sure to take vitamin supplements.
3) Live with it. Your metabolism slows down naturally as you age. You are likely to be thin well into your 30s, and then you will start looking great.

Hope that helps.

Talk to your doctor about it.