Food, lots of protein, balanced with a good mix of healthy carbohydrates and natural, healthy fats.
Mix that with iron. The kind you lift in the gym. You'll grow.
If you need more than you can get from whole food, protein shakes are an inexpensive alternative.
After that, if you need more, you can consider things like GABA, Glutamine, Creatine, Chasing your post
workout protein and creatine with dextrose.
Just start with one thing at a time. Do extra food first. Gradually add a bit more daily intake each week until you see yourself gaining or you reach the point where you just can't get in any more. For example Week 1 - add one extra meal a day. Week 2 - add another meal a day.
If you see yourself gaining watch closely to make sure it's muscle not fat - fat gains will come much easier than muscle gains.
If that doesn't work, then start adding protein shakes - in fact, you could just start those immediately by adding a preworkout
protein shake and a postworkout protein shake - that's two extra feedings each day going right to your muscles.
After that, come back and we can discuss other suppliments.