Hey bud,
I've got a D5000 (earlier model which isn't as good as yours) I've been doing photography about 6 months on a pretty continued basis. Best tip I can give is take your camera with you everywhere, the more often you shoot the better you will get with your camera and its settings.
I'd recommend investing in a decent tripod as this will be useful for any car photography as well as anything being shot in poor lighting conditions.
Filters are useful depending on application but I would recommend you invest in AT LEAST a UV filter for ALL your lenses.. it's much easier to clean a filter or even replace it than it is to replace a whole lense if you scratch it etc.
Further to all this what Matt has written above is spot-on. You want to read and practice all the time, there is plenty of info online for you and there are magazines in newsagents etc, if you intend on focusing on car photography read up on it.
I'll also add, if you're starting out don't get too down when you take shots and they don't look as good as magazines/websites etc. You will find these days ALOT of the shots you see are not taken straight from the camera, they are often touched up in photoshop and you can turn an average looking photo into a good one with little work so try make yourself familiar with photoshop if your not. Sometimes just reducing the image size, cropping out unecessary parts and sharpening it up can make all the difference.
Feel free to PM me with any questions and if I can't answer I'll ask my mate who does photography professionally (if you know anybody else who does photography ask questions and advice, deviantart.com is also a good photo-sharing place and some of the members are happy to help)
Cheers,
Steve