As a photographer, i always prefer portrait photography. the photographer who can take a good portrait can take any kind of photography (its my opinion no offense!). you don't have to worry about this matter until i am here to tell you the secrets of portrait photography :)

there is a huge number of lenses specially designed for portrait photography. if you are planing to buy a DSLR whether the brand(Nikon or Canon) is you have to keep in mind about the Auto-focusing motor. its a motor included in the Camera body which makes the lenses to operate automatically. the amateur photography cameras like Nikon D3100, D3000, D5000, D60, D40 and D5100 don't have this AF-Motor. the cheapest DSLR of Nikon that includes the AF-Motor is D90. so if you have the budget of buying D90 don't even think about others.

with most of the DSLR camera the kit lens is the 18-55mm zoom lens. which is specially designed of everything :) so actually its the lens that provide you everything but not perfectly or fully :) the main secret of making your portrait awesome is to make the back ground as blur as possible. to do this your lens have to be capable of taking images at a focal length of below 2. (f-2).

there are some portrait lenses for Nikon cameras and the best of them is AF-S NIKKOR 50mm f/1.4G (for the armatures). it will cost you around 500$. but if you don't have the budget then go for AF-S NIKKOR 50mm f/1.8G (i personally use it and its superb!), AF-S NIKKOR 50mm f/1.8D (its a very cheap lens but you will not get the auto-focusing option if your camera body don't include the AF-Motor.), AF-S NIKKOR 35mm f/1.8G. these lenses are really cool and believe me or not you will be amazed to see the portraits of these lenses. also these lenses are cool with low light so you don't have to worry at all.

another way to make a portrait extraordinary is to make the back ground as far as possible. distant objects become more blur. so the focused object become more noticeable.
so just by taking care of these few things you can also take extraordinary portraits with your armature DSLR.

Here are some sample portraits (Camera: Nikon D5100 lens: AF-S NIKKOR 50mm f/1.8G):

low light portrait without editing

Portrait with Motion back ground

close portrait unintentional

Distant back ground

Low light+ distant object

Night view with lighting objects in back ground

Night view with lighting objects in back ground

Close focus+Low light+ distant object