Camera accessories every beginner photographer should have?

I'm 13 and I just bought myself my very first professional camera. It's the Nikon D300. My fathers friend discounted this camera for me, as he bought a new one. Now, I've picked out a case. What else should i have? A cloth to wipe the lens, maybe? I have the charger, USB cord and lens cap, of course. I know there are filters you put on the lens to, for example, to lessen UV light. I want a couple of those. Do you have any suggestions? I don't want a new lens. The one I have is fine for now.

Tripod.

Your camera already has an internal UV filter. Using another one will not improve your photos.

Spare battery, Memory cards, tripod, cable release/remote.
UV filter, cpl filter.
The list is endless, so don't feel you need everything at once.

Neutral density filter might come in handy

Yes tripod. But a solid, quality tripod. And remote shutter release of some type.

Eventually you're going to want a new lens if you're serious.
TRIPOD. I would have that on my list right after the case.
A cable release cord… If you want to take pictures with long exposures.
Maybe some sort of manuel, you seem a little unsure about photography.

You're one lucky 13 year old… When I was 13, I could barely afford a Webkinz.

Don't waste your money on a UV filter. A polarizing filter is a good investment. Camera bag, tripod and then you should think about Photoshop.

Lenses
wrist/neck strap
Tripod

Micro fiber cloth.
You have every thing to start off. Get to know your camera, shoot like hell and things would follow.

A good neck strap.
The owner's manual.
A tripod.
A good basic book on beginning photography and composition.
A polarizing filter if you plan on shooting outdoors.
Next on my list would be a good dedicated flash.

A speed light
poralizer filter
remote control
tripod
extra battery