Confused about Metering and Auto Focus on Nikon Cameras?

I've been looking up stuff how to use my camera and I'm quite confused about metering and autofocus.

I know that matrix mode, takes into consideration of light in the frame of the photo and center-weighted is considering the center of the photo and spot is for a single dot in the center.

I'm confused on how do you use the metering, is it automatic do I have to half press a button for it to activate? Do can I use it in manual, aperture priority mode? Also when the Nikon camera focuses automatically will it meter the light or something along with focusing?

Added (1). Do I have to use it in manual*

Here is a link that will help you understand how to balance ISO, shutter speed and lens aperture.

You do not quite understand what center weighted metering does. What that meter does is use the center of the frame to account for 70% of the exposure and the remaining frame account for the other 30%, thus center weighted

Pressing the shutter release part way down activates the auto-focus.

The lightmeter is in operation when you turn on the camera. You will see this when you look through the viewfinder. There's a scale that shows under or overexposure as well as a -0- that shows that your image is properly exposed.

The problem here is you've jumped in too quick, first thing you need to learn about any new tech is how to use the supplied instruction manual.

The metering is automatic when you half press the shutter button or when the camera is switched on.

In Auto modes (and semi Auto modes like Aperture or Shutter priority) the camera will decide the exposure based on its light meter reading. What it is doing is 'centering' the light meter's position on zero - ie neither over nor under exposed. In these modes you can override the camera's decision by using the Exposure Compensation (EV) button to deliberately either under (-EV) or over (+EV) the shot.

In manual mode the light meter is displayed in the viewfinder and you adjust the parameters you want for the look and feel that you are trying to achieve in your image. Note that exposure comp does not have any effect in manual mode - you are adjusting the exposure. If you zero the meter you will be doing exactly what the camera does in Auto.

And yes, metering and focusing occur at the same time.

You really need to read and study the Owner's Manual for your camera.

Go to http://www.nikondigitutor.com/index_eng.html and choose your Nikon camera model and watch the video tutorial.