I want to do indoor head-shot in a low light situation. What is the right way to adjust the flashlight pointing angle to receive the best out of it.
PS : I use Nikon D3200 with 35mm f/1.8g lens and SB-600 flash light.
Bouncing flash is the way to go. Depending on situation and the surfaces we have around us, it may take a few practice shots to get it right. Given only one chance I'd choose the most sure way to bounce, but when time allows to search for it, my preference is this:
When there is a good deal of natural (or artificial) light present in the room, things become a bit more locational, I mean, more depending on the light source, the subject, the surroundings. The natural light could be given preference and flash would be a gentle fill light bounced from the opposite direction to the natural light. Or the natural light would be the fill light (downplayed with exposure settings) and flash powered up to become the main light. If there's plenty of natural light, then it might in some cases even be a place for direct frontal flashing with low power, or with a large diffuser in front of flash. With a lot of ambient light it also becomes a matter of having good or at least acceptable white balance in the lighting. If unsure, set exposure to diminish the ambient light and use the power of your flash freely.
A lot depends on the nature of the room you are in.
If the ceiling and walls are fairly light in tone and a neutral color then you can bounce the flash off the ceiling. The angle will depend on how far away your subject is as well as how high the ceiling is. Ideally you want to aim the flash at a point in the ceiling that allows the center of the beam to reflect onto the center of your subject. The ceiling will also act to disperse light throughout the room and back onto your subject. If the ceiling is rather high and the distance to your subject is small you may need to point the flash almost straight up. This can create problems with shadows around your subjects eyes and under their nose and chin. Placing a bounce card on the flash to direct a small portion of the light directly onto your subject will help fill out shadows caused by bouncing the flash overhead. If the ceilings are lower and your subject a little further you would need to adjust the angle down a bit.
If the ceilings and walls are darker or tinted then it gets more difficult. You will either have to settle for pointing the flash head directly at your model while mounted on your camera's hot shoe or more preferably move the flash off axis from the lens and point it directly at your subject while triggering it either wirelessly or with an off shoe cable. Any one of a number of modifiers can help to soften the light.
A good place to start if you really want to learn how to use a portable strobe such as your SB 600 is strobist.com
The key is to get the biggest area light source you can to avoid directional lighting issues. If there is a white or whitish ceiling, then you can point it up and bounce off the ceiling. Adjust the exact angle so that the appropriate amount of light falls on the subject.
If bouncing is not an option, the best bet is to use a diffuser and move the flash off camera if possible to get an indirect angle of lighting for more pleasant shadows. You can get fairly cheap diffusers like the Lumiquest softbox or a bounce reflector like the Lumiquest bounce devices. These will disperse the light over a wider area to soften shadows and make the image less harshly lit.
If you have a bounce card, you can also use that or something like the Lumiquest Quickbounce to send a portion of the light forward towards the subject while bouncing the remaining light off the ceiling. This can help with filling in the shadows while using a single flash.