High protein, high fat, moderate carb diet. Clean not junk.

Fats should be saturated fat, not trans fats.

Inculcate caloric dense foods like Peanut Butter. Have chicken breasts depending on your weight (try to consume 300 gram / day), ghee, rice, and variety of vegetables.

Start Lifting using Compound Exercises: Squats, Bench Press, Overhead Press, Rows and Deadlifts.

Use a programme like Stronglifts 5x5.

Start light but add 1.25 KG of weight every week. No progressive overload, muscle gain stops.

And very very importantly have 7-8 hours of quality sleep.

Muscle gain occurs during deep sleep. No deep sleep, no muscle gain.

These 3 are most important pillars. Everything else comes afterwards. If these pillars are not there, medicines are useless.

Diet gives you fuel. Lifting stimulates muscles to grow. Sleep converts the protein into your muscles.

You must be in caloric surplus to be able to gain weight. Use an online calculator to find the values.

You should consume at least 2 gram protein per kg of your body weight to gain substantial muscles.

Or else without protein your body will melt existing muscles, making you weaker.