If you are someone that is lean but does not have any muscle mass, then you will need to start bulking up if that is your goal, and a lot of times people will try to get you into dangerous steroids in order to fast track the process of bulking up, but it is strongly advised that you avoid using them, because a lot of steroids are not approved safe by the FDA for human use. So, for the sake of this article, we will be limiting ourselves to building muscle mass in a safe way.
You cannot expect to gain any muscle mass if you are not lifting weights. So, if you are not already, you need to add weight training in your workout routine and have a professional monitor your progress, make sure your form is correct, and slowly raise the amount of weights that you lift as well so that your body will not end up becoming used to a set amount of weight, because dig this, once your body becomes used to a certain weight and you continue to only use that weight, you will end up actually losing muscle mass again because it will shrink, so you will need to periodically increase the number of weights you handle.
You will need to seriously work on your diet as well. So, this means your diet will need to have a lot of protein, followed by fiber, and then healthy fats and minimal carbs. Protein happens to be the substance that makes muscles, so you will need to incorporate different kinds of proteins throughout your day, be it eggs, chicken, meat, supplements and powders and so on. Eating junk food and the like will do you no good, if anything, it will actually hinder the process of bulking up too.