How to focus with Nikon D5100?

A lot of tutorials I'm reading talk about focusing.Am I missing something? Doesn't it auto focus? The lens just zooms in & out.
Bear with me here. Just trying to learn.

Added (1). How do you turn the autofocus off?

You have a choice

You can use the auto-focus or manual focus.

Most people use the manual focus mode when shooting video because the auto-focus when using live view is much slower than it is with using the optical viewfinder.

When shooting video or using live view, you need to use an accessory like this

http://www.luminous-landscape.com/reviews/accessories/hoodloupe-3.shtml

You can focus automatically or manually with that camera, depending on the lens you are using.

With the Nikon kit lens on there, you can hold the hold the shutter release button halfway down and it will try to focus.

If you turn the auto-focus off, you can just turn the ring that is at the end of the lens to focus manually.

The zoom control is separate and you move that yourself - the movement you see is the optical train attempting to find focus.

Check your manual for selection of focus points and lock the central one (believe me, it's the only one you'll need). Put that on your subject and lock focus (your manual will show you how to do that), then re-compose.

I always have the exposure/focus locks set to activate on the initial half-depress of the shutter (when I'm not shooting manually), but this isn't for everyone