How to be a competitive swimmer?

i am 14 and gonna be a high school freshmen. i am thinking to join the swim team BUT i have NO experience in competitive swimming. i swim for fun. even at that im not a very good swimmer. as in i can't go to the middle of the deep end and just float there like MOST ppl. what are some things i can teach myself to become a swimmer like techniques and other info i need. i have til school starts which is august 29. Thank You :)