How to make the best of shooting in low light?

I want to take some night life photos. I'm USING A nIKON D3100. I have the basic stock lens that it comes with and it's built in pop up flash. I know I have no just but to use flash if I don't want any motion blur and what not. What's the best way to make good use of using the built in flash without it completely destroying the picture.

Depends what you are shooting…

low light is best for buildings - where you mask the people…

night shots with a pop-up flash aren't going to be that good… You need an off-camera flash…

pop-up flash is good upto about 10ft… Beyond that it's useless…

and for nights with strong street lights, you need to turn the ISO upto about 800…

If I could rip the built in flash off these things, I would. I suggest investing in a good flash and/or a tripod.

High ISO
Lens aperture wide open
Use a post production tool to reduce the noise caused by using the high ISO

Here is a sample of shooting indoors with a high ISO to keep the shutter speed high enough to reduce blur

There's no real good way. A flash at night is just that and there's very little you can do to mask it.
.
One way would be to use the flash but also a longer shutter speed. 2 or 4 seconds, maybe more, maybe less, for it depends on what your trying to do. Digital is nice because you can experiment and see your results right away and adjust accordingly. The flash will freeze life around you but the longer shutter speed will allow more of the night life light to come in as well as movement from cars, blurs of people, store fronts and even yourself if hand holding.
.
Some cameras allow the time exposure before the flash goes off and some allow it after. Each one giving a slightly different view of the night shot. This is called front curtain flash or rear curtain flash. If you had a small separate flash unit you could do it your self by firing the flash off anytime during the exposure, as long as the exposure was longer than a few seconds and you got your timing down.
.

The people that answered before me had some pretty valid suggestions but they really didn't answer your question. To shoot the best nighttime photos you can with your given setup is going to be a challenge. Your camera, the Nikon D3100 has an adjustable ISO setting from 100 to 3200. And in expanded mode, your ISO can be set as high as 12800. The higher the ISO the more "light sensitive" the camera's sensor is. As you raise the ISO your photos will also have more digital noise. Only you will know how much of this digital noise is acceptable for your purposes.

I would usually say to set your ISO to the lowest setting you could get away with but in this case things are a bit different.So, set your camera's ISO setting to the highest setting that you find acceptable. This will help you capture some of the ambient (background) light of the scene. You should also be able to turn down your pop up flash power with your camera settings if the flash is too harsh. This is called Flash Compensation. You will have to experiment to find what works best for the look you are going for. Once you get the look you want you can try turning down the ISO setting to reduce noise while maintaining the look you want for your photos.

Try to keep your shutter speed at 1/100th of a second or maybe even less if you can. This will allow more of the ambient light to affect your exposure. The momentary pop of the flash should freeze any motion of your live subjects. You will just have to be careful about using too slow a shutter speed as you will see blur of the ambient lighting if you have it set too slow. If you are comfortable using manual mode this will be more effective since you can also adjust your aperture to a wider setting.

There are so many other possibilities but for only using your camera and pop up flash, this is the best options I can suggest.

To be successful in shooting in low light you need to set your four main camera settings which are Shutter speed, Aperture, ISO and white balance, ISO being very important as a higher ISO can give you a faster shutter speed, choose an ISO of around 800-1600 to get a good shutter speed but also not as much grain. Adjust the white balance to a suitable setting, and your aperture to the widest possible. You can also use a Tripod to steady your camera.