Are you using a lightmeter device to read? or using the same lightmeter of the camera? You say that "it seems to have no effect on my photo" that means that you compare two photos with different settings? right?. Ok if I assume that you are using the same camera to read the light and you see no difference, probably is because the subject or the photo you are taking does not have too much contrast and you can try with spot, matrix metering, or center metering and hardly you will see a difference. For example, I have a D7000, when I need to use the spot reading because a strong back lighting, my camera has too a selective focus that synchronize with the spot reading and working with Program, or aperture priority, or shutter priority I have no problem whatsoever with the exposure. Try to follow the lesson that "FallisPhoto" is giving you and you will see the results.

You have to turn all forms of autoexposure off too. Otherwise it will just choose an equivalent exposure when you change the shutter speed or aperture. That would have no effect on exposure. The only changes would be in depth of field effects and motion blurring. If you want to change the exposure, you can change either the aperture or the shutter speed, but if you change the shutter speed, don't change the aperture to compensiate or you will get the same exposure. If you change the aperture, don't change the shutter speed to compensate or you will get the same exposure.

As for how metering works:

Most cameras have SOME kind of meter these days. These can be averaging meters, center-weighted meters, spot meters, or any of several other kinds. Most cameras have averaging meters. One thing they all have in common is that they will all give you an exposure recommendation based on a shade of gray called "18% gray." That is to say that they assume that anything you point the camera at is supposed to be this shade of gray. If it isn't, they will recommend an exposure that will MAKE IT so. Actually, the value can be anything; Minolta, for example, uses 16% gray, and some others use 17% gray, but I'm going to call it 18% gray throughout this article for the sake of simplicity.

For example of how a meter works, for the sake of simplicity, let's take a hypothetical situation in which you are shooting two different nudes, identical blonde twins, each of which has a different colored backdrop behind her. One nude is standing in front of a white backdrop. The other is standing in front of a black backdrop. This is a simple situation because you essentially have only two values (range of light to dark) in this situation: the nude and the background.

So let's start with the nude with the white backdrop. You take the photo, develop the film and make prints. You will find that you have a photo with a gray background and the model is far too dark. Your film has been underexposed and you can't figure out why. You set the exposure the way the camera said to and it still came out too dark. What happened?

Well, when you turn on your camera's light meter what the meter sees is the whole composition. It's just a machine that measures light. It can't recognize objects or what it is looking at. It doesn't differentiate between model and background, but what it does do --immediately -- is decide that the composition as a whole does NOT average out to 18% gray. Even though Caucasian human skin is roughly the right shade, it also takes the background into account. It will recommend an exposure setting that will make the background and subject darker, in order that the composition as a whole will average out to that 18% gray it wants. This results in underexposure.

Now let's photograph the model with the black background. Again, you aim the camera, adjust the exposure as the meter tells you to do, and take the photo. You develop the film and make prints. You are perplexed when you find that the background is still gray, but this time the model looks snow-white. Your film was badly overexposed. Again you are left scratching your head, and now you're wondering if the camera is broken.

The camera and the meter are both working fine. What happened is that the meter looked at your composition and found that once again, it didn't average out to 18% gray. This time it was too dark, so it recommended an exposure that would make it average out to the "right" shade of gray. It overexposed your film, to make the composition lighter.

As you have probably figured out by now, the camera will only give you an exposure recommendation that is absolutely correct when the composition really is 18% gray. Unfortunately, in real life, this doesn't happen very often. What you have to do is recognize that this is happening and take it into account. You have to make adjustments for it. As you gain experience you will find that you can do this by eye, but fortunately for the beginners, there are three relatively easy ways to get around this: spot metering, incident metering, and gray cards.

In spot metering the meter only looks at one small area of the composition. All you have to do is find an area in the composition that is close to 18% gray and meter the light on that. In our hypothetical nude photography scenario, the model's skin is pretty close to 18% gray, so this won't be a problem. What do you do if there is nothing in the composition that is the right shade of gray though? What if your model is an albino? What if she's black?

Well, there are two more ways of getting the right exposure. One is to use a handheld incident meter. Instead of measuring the light reflected from the model toward the camera, these measure the light that is falling on the model directly. Instead of standing by the camera and metering the model, you stand by the model and point the meter at the light. This way there is no possibility of the meter getting confused by different colored backgrounds, because value of the background and the value of anything else in the composition never comes into it.

By far the easiest and most convenient solution though, is to use a gray card. A gray card is simply a piece of pasteboard that is colored 18% gray. They're one of the few things in photography that are cheap. A gray card gives you something that you can hold in front of the camera that actually is the right shade of gray. You set your camera on manual, hold up the gray card, set your exposure, then set the card aside, focus, and shoot, confident that you will get a perfect exposure.

Metering per se does nothing to the exposure, and is not affected by the AF points. It is used to measure the amount of light in a certain scene or spot, as to see if it's exposed properly. If you have it in spot, and you point your camera at a tree, for example, it would show +0 if the exposure is neutral. Assuming it's a cloudy overcast day, if you point at the sky, it will show +3 or similar, which means the sky is 3 stops brighter than the tree, resulting in a properly exposed tree against a white background. If you are in Av or Tv mode, you can set the Exposure Compensation to expose the scene as you want; so an EC of -2 will reduce the camera's calculated exposure by two stops.

Oh man, I am taking a college photography course and metering is still just as confusing to me! Hope you have better luck than me!