Sport shoots at night

I just got a Nikon D5100 and I need to take pictures of football (american) games at night.
I have tried several settings but the pics are too dark or too blurry.
If anybody has any advise on their experience. Please, I need your help.
Victor

Hi and welcome...I shot soccer at night once and used my 50mm lens set at F1.8....ISO800...Shutter at 250th of sec..

Shot in raw+basic and was able to get some good shots....

Like pedroj said, you need a fast lens f2.8 or better, crank up the ISO and you should be able to get some decent shots, anything slower and you'll end up with shutter speeds so slow you will not be able to capture the action, they will be blurry.

Would you not need something with a bit more reach than a prime such as the 50mm?
Maybe the AF-S 300mm f/4 ED-IF (~£1000) or maybe even a AF 180mm f/2.8 (~£635)