Eat and lift. The program doesn't matter too much; there are many decent ones. Anything with a focus on the main barbell movements (squat/dead/bench/overheadp

Eat and lift. The program doesn't matter too much; there are many decent ones. Anything with a focus on the main barbell movements (squat/dead/bench/overheadpress) with some basic progression scheme is likely sound. strengtheory.com is a great source for info; Greg is legitimately strong and actually has some basic science understanding, unlike 99% of 'fitness nerds' who claim to. Changing your diet will likely take as much effort as the training. Feel free to use an unusual diet to 'learn' to eat more calories. Even if you have to use lots of milk, bacon, or whatever is tasty and easy to get down, your body gets used to the calorie intake and transitioning to a reasonable diet will be easier. The real challenge is in habit formation. Try to make it easy on yourself and use whatever tricks you can to stick with it.