Honestly dude, you probably already have the best lens. The best lens is the one you already have.
But all you have told us is that you have a D90. Reaching out on a limb here, you probably have a 35-80 vr lens that came with your camera.
But the most important piece of equipment you can get would be a... (wait for it)
A MONOPOD.
You are going to be taking a lot of pix. Most of them will be culls. How-ever, with a monopod to help steady the D90 those culls can become keepers.
Also, if you save the images at the LARGEST file size, you will be able to crop a boring picture into an exciting action shot.
I don't know what your budget is, but it is far better to sharpen your photographic skills rather than expand your gadget bag.
Just my 2 cents.
Pete