How can i build a round, muscular, 'bubble '?

This is the situation: I'm a 19 year-old male college student. I have been active my entire life in sports and other activities, and i have never been a big eater, so i have a slender/athletic build. I have really great leg muscles and i'm quite lacking in the upper body muscle department (blame 15 years of soccer). However, I have a pretty bad hamstring imbalance (also from soccer), so i have giant hamstrings (along with quads and calfs) but itty bitty glutes. Not only is this a problem because I'm tired of having a flat ***, but i also know is can effect my posture and lead to back issues later in life. I'm mostly just concerned about my image though. So i want to know how to build muscle on my , and tone and lift to too. Any advice is helpful, e.g. diet advice, excercises, weight machines, workout machines, etc. I go to a fairly nice college so i have access to a very large and well updated gym facility. Thanks ahead of time for any help. :)