www.nikonians.org.
I think you have to pay $25 now, but it's well worth it. You can also Google specific topics to help you out. Given the D300 doesn't have 'canned' shooting modes (eg portrait, landscape, etc), you should definitely Google 'exposure' to get the basic balance of shutter and aperture choices, 'aperture priority', 'shutter priority', and 'programmed' shooting modes. 'P' is the place for you to start as it picks apertures and shutter speeds for you...it's as automatic as you'll get. However, no experienced photographer would ever use 'P'. Most people shoot in 'A' where you choose the aperture and the camera chooses the shutter speed. 'S' is the opposite with you selecting a shutter speed and the camera picking an appropriate aperture. 'M' is full manual where you pick both settings. ISO is the sensitivity of the sensor...higher will allow faster shutter speeds in lower light, but the cost is more grainy noise and loss of contrast and color. Frankly, there is just too much to explain here. I think one of the books will be your best option by far unless you already know the basics. From there, the sky's the limit. The D300 has great customization that you can really tweak to your preferred style. And I'd argue any day that the D300 is still the best DX format camera on the market (for very technical reasons).
There's a book called "Mastering the Nikon D300" that would help you (the tiger photo starting chapter 8 is mine...though I have no financial interests in this book), and the Magic Lantern Guides are good for beginners too. If you've never used a DSLR before, the D300 will definitely be overwhelming at first.