I congratulate you on your endeavour to strive to be in competitive swimming. Besides having the grit and determination, one needs to work on speed and agility, physical and mental fitness and building the fire to win!
For the first aspects, you should work towards building your stamina and strength in heart and lung or cardiovascular fitness; besides doing this in swimming, it might help to include some /brisk walking/running on land, IF you have the time. Or else, continue swimming to get to your optimum levels of performance.
Besides this you would have to take care of strengthening your body by doing some muscular strength and endurance exercises, in the form of perhaps, push-ups (for the upper body), squats (for the lower body) and abdominal and oblique crunches as well as back extensions for the mid section.
You should also include flexibility workouts like stretching everyday to keep your body supple. Mental fitness can be enhanced by some positive thinking and attitude building exercises. You will also find that by doing balance and co-ordination exercises, you are gaining an overall fitness perspective.
As far as your diet and perfect body mass goes, get in touch with a nutritionist. All the very best.