Nitin, Good move! You will love the D300, and I don't think it will take long for you to do very well with it. I use variable ISO, lower limit 200, upper limit 3200, and shoot in 14-bit RAW. Sometimes aperture-preferred auto, sometimes manual. Matrix metering for most situations, spot for high-contrast situations in which you want to choose to expose for a middle to high middle tone.
I would suggest reading about the Zone System. This will increase and refine your understanding of exposure and its nuances.
Keep your shutter speeds as fast as possible--this is why I suggest using variable ISO, and not sticking with a fixed low ISO like 200. If you shoot at ISO indoors without flash, the you will be using very slow shutter speeds which are difficult to hand hold.
The D300 allows you to set a minimum shutter speed--I would suggest 1/15 of a second. You need to make sure to hold the camera very steady, and SQUEEZE the shutter release--not jab at it.