What lens do I need for nikon camera for?

I need a lens that shoots a person from head to toe close up. I've been doing research and people have suggested the 35mm or 50mm 1.4f. What's the difference between 50mm 1.4 and 1.8? Should I get any of these lenses for what I need?

No idea

If you have to stand close to the person and still get a full body shot, you need to set your 18-55 mm lens to 18 mm and then move toward or away from the person until you have what you want.

However, there will be some distortion since the best lens for shooting a person is a medium telephoto lens, so you will have to back up until they fill your cameras frame.

It is your choice. Either back up and use your lens at 55 mm or get close with distortion and use your lens at 18 mm

You don't mention the lens which is on the camera now, or what lens came with the camera. I think that any normal lens which is at the end of a standard zoom lens (18mm-to-55mm)can capture what you want.
If you want to shoot head and shoulder portraits of young people then the 50mm lens is fine. Or the 50mm position on the zoom lens which is on your camera.

Now f/1.4 or f/1.8 lens. I would use the f1.8 lens outside and inside with lots of lighting. However the "faster" f/1.4 is more desirable for "available light" pictures, or rather shooting under existing lighting. I would not recommend the 35mm lens for people pictures that you desire to get close up of these people.
However, I hate to bust your bubble, but my recommendation is to save you money until the specific need presents itself. If you had enough experience you would be familiar with the difference of f/1.4 and f/1.8. I hate to see people spend their money with out sufficient knowledge of what is needed to accomplish what they want. Just asking "us" is not the real answer, because we all have out bias's.

The difference is angle of view. The more mm the lens has, the narrower the angle of view. For full-frame dSLR's, 50mm is better. For crop sensors, 35mm is better. F/1.8 will do but if you afford the f/1.4 version, you will be able to work more in low light conditions and make blurring backgrounds easier. The lower the f/number, the larger the aperture size is.