Here is the WIKI source on just that.
http://en.wikipedia.org/wiki/Skateboarding#Culture
Not being a skateboarder, it's hard for me to know what you want your main points to be. So, think of three things that point to it being its own culture, such as:
PAST IMAGE
CURRENT IMAGE
MEDIA INFLUENCES
WIth your three main points construct a sentence. "Skateboarding has taken on its own culture over the years despite the past image of rebellious teens taking up the pasttime. The current image is leaning more towards sport image and fashion image. Movies and songs have also played a part in the promotion of this new culture."
*just an example...I am not up on the facts of skateboarding
GOOD LUCK
