Good shots!
When using manual mode, really the key here is to in the first instance, know exactly how to control the key elements of the exposure, in DSLRs this is pretty much:
ISO level
Shutter Speed
F-number
In low light situations it's helpful to have a low f number (e.g faster exposure speed) in order to get as much light in as quickly as possible. Why? This negates appearance of blurry shots.
Once you have an intimate understanding of playing with the above 3 settings the rest is really down to you... play.... experiment.
If there is a particular type of effect you're looking to re-create, then there are a swathe of online resources for this as well as good books. You can get really good guides for all Nikon DSLRs, I can't remember the publisher name though all I can say is, the cover art looks very square and blocky and cites your model of camera on the cover
You can find these in Waterstones all day, so go and find one and snag yourself a Coffee at the same time
Low-light is always more challenging. Cranking up the ISO product noticeably more grainy images and utilising an in-build flash often ruins a shot. As much as possible I avoid flash in mine, opting for optimum selection of ISO, F-number and shutter speed, although you can only go so far without a tripod and only so much further with one.
When using a flash, I recommend lowering its intensity by setting the delay on it (you'll need your manual / reference book for this.) This essentially advances the fire of the flash more and more so that you picture the fading light rather than the full intensity of the light.
Another option is using one of the better external flashes and using a cowelling over the flash to decrease it's intensity.
The shots you took above would not, in my opinion, benefit from a flash, I would perhaps increase the ISO a notch or two and play with the shutter speeds a little to fine tune.
Great pics, lovely looking car, I wish I'd gone for a white one now, it looks amazing.
Hope that helps,
Alex
