What can smoking 1 pack of cigarettes do to you?

I know I'm stupid. I'm young and I did it. Now I regret smoking a pack in a month. Tell me what will happen to me now! I KNOW SMOKING IS FU$%ING STUPID.... I'M QUITTING NOW. I'M ONLY 15.. It will effect and sports? just a pack? Good details please.. And how to quit smoking?