How to use AE-L/AF-L button?

I have recently bought a Nikon D 800 Camera I also own a Nikon D5100 Camera. On my D5100 I can point the camera at a light, exposure will adjust, I can press the autoexposure lock button and the camera will lock the exposure I can move the camera and point at anything and the exposure will stay locked so long as I hold the button. On the D800 this does not work. I have pressed and held the button and simultaneously half held the shutter button and the exposure will not lock I've looked online I've checked the settings I can't figure this out. Can anyone tell me how to properly use the autoexposure lock button on a Nikon D 800 Camera?

This is covered in some detail on page 128 of your manual. Essentially, first you autofocus by pressing the shutter button half way down. Then, while holding the shutter half way and keeping the focus point on the subject, press the AE-L/AF-L button. Then, holding the AE-L/AF-L button down, recompose and shoot. There are various visual feedback items that are enumerated in the manual. Did I suggest you should read the manual?

All of your answers are in the manual.

Personally, my preference is to assign the rear button to do the focusing and turn it off at the shutter button. Then use manual so I don't have to worry about exposure lock.

You meter with the subject at center using spot or center-weighted then suddenly want to recompose. To avoid any changes to exposure, you lock it using the AE-L button.

It's the same thing with focus. Most old-timers are used to a single focus point at dead center. They focus with the subject at the middle then recompose before shooting. Again, you can use the AF-L button to lock focus so that it won't change when you recompose.

Most dSLR's have both options in one button only. There's an option to lock both exposure and focus at the same time. It's deep within the menu system. If you shoot in manual mode, you won't have much of a problem with AE-L. If you focus manually, you won't have problems with AF-L either.