For your 6 pack to show nicely, you need to be around 8% bf.
What will help you lose that last bit of fat is to add some muscle to your frame and boost your metabolism.
A gym is nice but all you need is one of those playgrounds with some steel in it. When I can't hit the gym I find one of those and get an awesome workout.
With just a few bars and basic playground junk you can do push-ups, chin-ups, handstand press, dips, horizontal chinups, etc. etc. be creative but careful and you will do fine. Try and find exercises that you can do no more than 10 of and do 3 or 4 sets of those with a minute rest in between sets. Do this 3-4x a week. Eat your protein, (about 1g per pound of your body weight per day). Remember you are still growing so eat plenty, stay away from sweets and fatty stuff and you will get what you want.
Don't give up, and don't miss a workout.