The tomboy style can be easy to accomplish for some and harder for others the key to it if you're having a hard time look at what more preppy boys wear then go from there. Blazers, looser pants (bit of sag doesn't hurt just don't go to low), graphic tees, collar shirts, and hats are just something to add to the look. Now tomboy style isn't exactly huge among girls and can be at times looked down upon in say a high school. (sorry going on a tangent for a second) When I was younger I was a total tomboy (we're taking k-6th grade). I dressed in sweats loose teeshirts, and I never cared to much about what I wore. Many girls in my grade were already discovering makeup when I was discovering that pink was a color girls wore (I was more a fan of blues and green). In seventh grade my style began to change more to the casual preppy but I still love the tomboy look. Some people this is their style and they look amazing, and some of us can't pull it off (don't try to hard it won't work).
I may be posting many tomboy style photos so I thought I'd just give you all my opinion :)