I am pretty new to DSLR exposure setting with Flash, when I do not use flash, I know how to set Aperture ans Shutter speed to get correct exposure, but when I turn on flash, the metering system can not take that flash light source into calculation to give me current metering value(it is still based on current light condition).
I wonder how to set them to get correct exposure? In A or S mode, the camera may help a lil bit by adjusting the other parameter(mine is D7200, its buildin flash has TTL, so in A, the s changed, and vice versa), and does that mean I need to give couple of shots to manually figure out the exposure in M mode by reading the meta info of picture taken?
my scenario is night portrait with limited street light
There's two things that you may want to do here:
1. Light the scene with 100% flash using TTL
This question has pretty much been answered here. The gist is, set you exposure to just about anything you want (underexposed) in M and the camera will fire the flash to get a proper exposure. The more under-exposed your initial setting, the more powerful the flash - up until the point that you max out your flash's power.
2. Use the flash to add a bit of extra light while relying on the ambient
Let's say that you set your settings above to be 5 stops under-exposed and then took the shot. The photo would be 100% lit by flash. As you adjust your settings toward proper exposure (4 stops under, 3, 2, 1) the camera will use less and less flash to compensate, thus "mixing" the ambient light with the flash. This is often referred to as mixed lighting.
Using a flash to soften shadows that might be caused by a harsh sun or streetlight, for example, is called Fill Flash.