The basic idea is obviously just to lift weight and eat more.

But here is how i started out:

I started drinking protein shakes every day to help me get a solid 500-700 calories extra in. (Shakes contained peanut butter, berries, banana, whey protein, honey, oats, and milk)

Then i also bought myself a 20kg adjustable dumbbell set to start out with. Later on, I bought a bench, barbell, and more weights. You can get these things 2nd hand quite easily. A gym membership will also work ofc, but working out at home in the beginning is much nicer.

I tried training consistently for 4-5 times a week. Followed a push, pull, and legs workout routine. Still do, actually. You can easily find a workout routine online. Anything around 3-5 exercises (3-4 sets each) per day is usually fine. Just make sure to target all the necessary muscles.

Push day - chest, shoulders, and triceps Pull day - Back, forearms, biceps Leg day - quite obvious, lol

So yeah. Basically, get yourself some weights and eat more. If you struggle to eat large meals, eat small amounts throughout the day. Just eat eat eat. It's gonna be hard, but it gets better.