D90 New lens help

Hi all,
Sorry if I am just being air headed or just an utter noob regarding the lenses but I really can not work out what is best.
I found this post "http://forums.dpreview.com/forums/thread/868100" and it makes a bit of sense, but I am still unsure what kind of lens that I need to get for taking sports shots of surfers.
From what I have worked out I think I would need to get a lens that has two large numbers eg 200-300mm or similar.
Nick
D90, 18-105mm