Help regarding DSLR?

I'm buying Nikon D5100 (body) only. Please tell me which lens should i buy for great image especially low light image.
If lens produce great low light image then 100% is going produce great portrait, landscape, general image.

A good general lens for low light and usable for landscape and portraits would be a 'standard zoom' at constant f2.8 aperture - focal length around 17mm to 50mm. But not cheap, about same price as camera body. Specifically for portraits and low light would be 50mm f1.4, but not good for wide-angle.

We fit lens based on the dimensions of the sensor. The Nikon D5100 sports a sensor that measures 15.6mm height by 23.6mm length. Knowing this, we can calculate the diagonal measure of this rectangle, it is 28.3mm. This is important because if we mount a lens with a focal length that is approximately the diagonal measure, the angle of view will be "normal". Normal means, not telephoto and not wide-angle. A wide-angle lens for this format starts at 70% of normal which works out to 20mm or shorter. Telephoto starts at 200% of normal, that's 60mm or longer.

Now this camera is normally sold as a kit that includes an entry level zoom lens. That lens has a variable focal length and is known as a zoom lens. The range of this kit lens is 18mm through 55mm. Note, the center of the zoom range lands at "normal". The max zoom is the beginning of the telephoto range, the minimum is the beginning of wide-angle. This is no accident. The focal length range was chosen to provide a versatile starting lash-up.

Unless you have deep pocket, you would be wise to start your photographic adventure with the body fitted with this kit lens. After you get some practice time under your belt, you will be in a better position to choose what will be the next lens you need to enhance your new hobby. Meanwhile, trust Nikon's choice and buy the kit.

The lens will depend on the type of subject you will be photographing. Shooting landscapes will require a different lens than shooting a sporting event.

There's no great "does it all" lens.

There isn't one lens that's "great" for everything. That's why there are so many different lenses available.

For "low light," you'll want a "fast" lens -- say a 50mm f/1.4.
For portraits, probably something a bit longer but still "fast," say an 85mm f/1.8.
For landscapes, often a wide-angle lens (around 14-18mm on your camera), and it really doesn't matter if it's "fast" or not.
For "general" shooting, a "super-zoom" might be fine, or at least something in the 18-135mm range.

Your about to find out just how expensive the Nikon system is.

My advice would be to buy the kit lens along with the camera, buying it this way gives you discount, it would add around £100 to the body price. Use that for a while and you will find out if you NEED more aperture or a longer reach or the Macro lens.

The wider aperture upgrade from the kit lens is the Nikon 17 - 55mm f2.8, but that's over £1000, good though it is, it's the 'workhorse' lens for most Nikon users.

Or the 24 - 70 f2.8, but that's even more expensive and aimed more at cameras with a full frame sensor.

Pentax is a much cheaper system buy far to buy into. You can use any K mount lens, lots available on eBay for sub £100, all will be image stabilised as it's built into the camera, not an 'extra' you have to buy with every lens. Image quality is at least as good as Nikon. (loads of thumbs down from Nikon owners, but it's true).