How do I change over the summer?

Okay..so I'm 15...I'm a guy..I just finished eighth grade...and this summer I want to change. I have brown skin..black hair..and glasses...the typical nerd. I used to be overweight but I lost weight in fifth grade and became skinnier. I'm not fat...but I have a flabby stomach and no muscles on my arm/stomach area. I only have muscles in my legs because I run cross country. I want to change. When I return..I want to be the guy that most girls have crushes on. I want to become muscular and get a six pack. Also..I have a highish sort of nasaly voice...I want to deepen that into a man's voice. Please help me! I would really appreciate it! Thanks!