Okay, 148
pounds on a 6'6" would look emaciated, in my opinion.
I'd say what you want to do is start resistance training. Build up some muscle and you will start looking good and fill out those tight shirts the way you want to.
I don't recommend going by
BMI for
body fat, or for anything else for that matter.
Instead go by how you feel and how you look, and how your clothes fit. Don't let a table tell you how much you should weigh.
But yes, for exercise I heartily recommend resistance training. Go do some squats, pullups, bench presses, that sort of thing.