Sports photography is all about shutter speed. You need a fast shutter speed to freeze the moving athletes so you don't get motion blur in your photos. But fast shutter speeds don't let much light through to the sensor, so you have to set the ISO and the aperture (lens f/stop) to settings that gather and receive lots of light so that your pictures aren't too dark. I may be telling you things you already know, but I'll cover as much as I can just in case you're new to photography. There's too much to cover here, actually. If you don't understand how the different camera settings affect exposure and your images, I'd recommend you buy and read this book:
http://www.amazon.com/Understanding-Exp ... 0817439390 It will show you how the ISO, f/stop, and shutter speed settings work together to achieve a proper exposure. They work as a team. When you change any one or multiple settings, you will change your exposure.
In short, for sports you need the fastest shutter speed you can get while still getting enough light for a good exposure. During day games, it's easy. But in the dark, especially in the typically poorly-lit 6man stadiums, it is next to impossible sometimes. Higher ISO numbers make your camera more sensitive to light, so your pictures will appear brighter. BUT....high ISOs mean a lot more grainy noise will appear in your images. You want to shoot the lowest ISO you can while still getting a shutter speed fast enough to stop action.
Ideally, you need shutter speeds faster than 1/500th of a second to freeze the action. 1/2000 during daylight is awesome, but you can't shoot that fast in the dark. Your pics will just be dark, because there's not enough light getting through. You can get by as slow as 1/250, but you'll still have some slightly blurred shots when you get that slow.
I looked at one blurry pic in your gallery, and it was shot at 1/60.
https://picasaweb.google.com/1068623708 ... 5590221026 That's way too slow, obviously. That's why it was blurry. (No offense. Just talking photography.) It was shot at ISO 800, which would be nice to not go above. But in the dark, you will have to shoot at at least ISO 3200 and f/2.8 on the aperture setting for the lens. I see your lens has a maximum aperture of f/4-f/5.6. Smaller f/stop numbers equate to larger apertures. You need to get a lens that is f/2.8 or better. Unfortunately, they're not cheap. You don't have to spend the extra money to get one with image stabilization though. Just get a monopod to hold your camera steady. It'll save you about $1,000. But you do need to spend some $ on an f/2.8 lens if you're going to have a prayer at night football game pics.
Once you have a lens that will allow you to set the proper settings for sports in the dark, then you need to look at a couple other settings. I'm a Canon guy, so I'm not familiar with what they're called on a Nikon. Check your manual, and you'll find what I'm talking about though. Look at your auto-focus settings. Find the setting that allows the auto-focus to follow moving objects while remaining in focus, and set it for that when shooting sports. Then find your focus point settings and set it so that it is using only the center point or a few points in the center rather than allowing the camera to use all of its focus points to find something to focus on.
Third, look in the manual (or Google) "back button focus" for your camera. You can set the camera so that one of the buttons (near where your right thumb will be) on the back of the camera will make the auto-focus kick in. That helps, because you can just hold that button down while you're following the action, and then you can click the shutter button whenever you want to take a picture and it will be in focus.
There's more to learn and practice, but these things will get you off and running very well. Hope that helps.