You'll need to do a balanced weight lifting program. For every exercise there's an agonist, and, in order to maintain balance (so you don't have, say, the unbalanced upper body of a swimmer) balanced weight lifting is key. Weight lifting will also strengthen your bones, and increase your lean muscle mass, which increases your metabolism, as you get OLDER, and well as aid in injury prevention by strengthening your connective tissues, as well.
Lifting will help you with speed, and execution, in any particular sports activity. If, let's say, you're serious about martial arts, lifting will be a mandatory in your training for the power that it will add to your moves in that sport, and how it helps to prevent injury. Those functions are both neurological and muscular, and the lifting will improve both, making your nerves able to fire faster, and HARDER, and by giving your more muscle to generate more power to be a more serious competitor in that sport.
Any person serious about fitness, or who takes his sport seriously, will always include resistance training, in a balanced fashion. You need to begin that program now.
Not doing so really kind of signals a lazy athlete / someone who doesn't understand the big picture. You'll want to move forward with bringing your activity to the next level by including resistance exercise.