So you have, or are thinking about getting a DSLR. The logical question that arrises is: what lens do I get? or Do I really need a lot of different lenses? Lens choice really is one of the most fundamental things in photography.
Let me explain why. PHOTOS ALWAYS LIE!!!! the lens plays a tremendous roll in how they lie. Why this is important - you have heard the saying "the camera always adds ten pounds" well not really but wide angle lenses make you look fatter, telephoto lenses make you look skinnier, 35mm-80mm (assuming a full frame sensor - or correction for crop factor) sees the world close enough to the human eye that we don't really notice the distortion, under 35mm is considered wide angle. I can make a room look larger in a picture by shooting it with a wide angle lens - I can, and will whether I want to or not, do the same to a person, if I shoot them with a wide angle lens. Wide angles also let you do really interesting things with forced perspective, if you lay a wide angle camera on your chest and shoot towards your feet, in the image your feet will look abnormally small and far away. On the other side of this, fashion models are generally shot between 200-350mm to knock off a size or two or even three with the distortion (compression) of the lens.
So what about f-stop, sure it controls light - but you should control light with the light - let f-stop be reserved for depth of field. For most things you can get a nice shallow depth of field with f4. Both Canon and Nikon have an inexpensive 50mm f1.4 and Tamron makes some nice 2.8 zoom lenses (I have the 28-75mm f2.8, I really like this one for parties) which hold 2.8 across the zoom. Most zoom lenses have a variable aperture that is it will be something like f3.5-5.6 depending where you are in the zoom. Always remember the closer to your subject the shallower the depth of field, use this to your advantage when you need a shallow depth of field. The pattern of the bokah is controlled by the shape of the aperture. If the lens has a really scalloped kind of bokah you can correct it by using a piece of paper and a hole punch effectively putting a round aperture in front of the lens. Or you can make it heart shaped if you want - or even spell words.
So how all of this applies to what you are asking - If you can only get one lens then I would advise something like the 18-200mm zoom (in Canon this comes as a variable f3.5-5.6), which is my goto lens on my Canon EOS 7D, as it will give you the widest set of options - but in general for portraiture I would say over 80mm in a prime and an f-stop of 1.4 if you can get one in your price range (I am not that familiar with Nikon) and zoom with your feet. Once you go over 80mm the lens distortion starts to be more pleasing to how we want the world to view us - skinny! You can do the close ups by moving close to the subject and if the room allows the full body shots by backing up. You can then be very specific on the part of the subject and how much of the subject you want in focus by using the f-stop to control depth of field and how and where you want the light by using your lights (and for this you can start with CFL work lights from the hardware store just get all the bulbs the same temperature and set accordingly on your camera) and light modifiers (most of these you can make yourself).
Note - how you light a subject will also effect how they appear in the photo. Lets say the subject is turned 45 degrees left and the light is shining from that side directly on them (Broad Lighting) it will make them look bigger and is generally used for lighting men. If the light cuts across form the other side of the frame then it is (Narrow Lighting) and makes them look thinner - generally used for women.
Now the advice I give is based on the assumption that you want to do photography where the subject "looks pretty". But say you want something a bit more abstract or alien - then you might want to go below 35mm and maybe even into the fish eye range. Though if you are below 35mm and moving in on the lenses macro focusing (assuming the lens has it) then you will start to get a lot of distortion in the image even if it is not a true fish eye lens. But this gets back to PHOTOS ALWAYS LIE - think about how you want the photos to lie, that will dictate the lens or lenses you should get.
My goto lens on my Canon 6D is the 24-105mm f4 L which lets me work from wide angle to telephoto with out switching lenses and gives a decent bit of flexibility with the DoF. I also carry the 50mm f1.4 which on the ACP-S sensor of the Canon 7D works at close to 80mm. I also have a pair of the 28-135mm f3.5-5.6 which is sort of Canons better kit lens depending on the body you are looking at. Not a bad lens either for flexibility though on a cropped sensor (as in the Canon 7D or Canon 60D) you have lost the wide angle aspect of it.
I hope this gives you an idea of the things you should consider when choosing a lens. But also remember you get what you pay for. There are plenty of cheep lenses - but they will give you cheep photos as they are not made to a very high standard. An example is Canons 50mm f1.8 which only runs about $100.00, but when you filter your photo through it the loss of quality over even the 50mm f1.4 is obvious let alone the 50mm f1.2 L.