I'm 22 I'm normal in height almost 5.9 but my weight is around 56kgs, I'm looking lean. How can I be stronger?
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Right now, you're underweight, so the first advice has to be that you should eat more. For this you might want to track everything you eat for a week or two, to find out how much you do eat right now. From there, you will most likely want to increase your calorie consumption to provide a caloric surplus (depending on what work/sports you do that could be 2500-4500kcal per day).
Next you will want to start a training program. I like to suggest Starting Strength, which is a weight training routine. If you have access to a studio, I'd go with that, since
it works and it works fast.
If you only have your home, you might think about a bodyweight routine. You won't need any equipment for that and you can add in exercises whenever you want to (lunch breaks or whatever).
Whichever you choose (there's much more out there, so maybe you choose a completely different program) you have to eat more. You cannot build muscle or strength without a caloric surplus.
You also might want to get rid of the idea of a six-pack for the next 12 months. When you build muscle, you will inevitably build fat, too. You can lose that fat later, but in the beginning it's better to eat much, train hard and worry about the fat later.