Your Nikon D90 digital camera is fine for Sports Photography. There are better cameras available and also better lenses, but they are only going to give you better quality pictures with the correct technique.
It sounds like you need to learn some of the techniques involved with taking digital sports photography. The steps are quite easy to learn, but take a while to master.
I have a section related to Sports photography on this website. Basically you need to change a few settings on the camera, such as shutter speed and maybe try burst mode. Try having a look at this page for some tips.
If you still can't get it to work, then add a comment to this page with the details of the shot you are trying to achieve and I will talk you through it step-by-step.
For any other photography related techniques, my business partner Amy Renfrey has a great ebook called Digital Photography Success, where she teaches you how to take great portraits, landscapes, nature, etc.
She will soon have her own page on this site to answer questions related to photography techniques. Watch this space!
