How to achieve perfect exposure when taking a photo on a DSLR?

I've recently purchased the Nikon D5000 DSLR with a 18-135mm lens. I absolutely love it but keep coming across the same problem.

When taking photos I can never seem to achieve perfect exposure and the photo is always to dark. For example, if I wanted to take a picture of someone running, I understand that I would need to set a fast shutter speed and a wide aperture, however when zoomed in, the best aperture I can get is f/5.6. Not enough to get good exposure.

I'm new to photography and would really appreciate any help to get me started.

Thanks

Added (1). I'm shooting in Manual mode and I do pay attention to the light meter which forces me to lower the shutter speed. I've adjusted ISO constantly to see its effect, however there seems to be absolutely no difference between an ISO of 200 compared to 3200…

Bump up your ISO.

The lens you have has a max aperture of 5.6 when zoomed in, so either lower your shutter speed if the image is too dark, or raise your ISO.

It would be good if you could mention the "mode" you are in while taking the picture (like aperture priority, shutter priority, program, etc).

You are right about your lens - it is a variable aperture lens and the largest aperture it can have at 35mm is F5.6. On a sunny day, with most ISO settings, that should actually be fine. If you haven't experimented with ISO, the sensor's sensitivity to light, try increasing it gradually. One downside of using a high ISO is the amount of noise it can introduce into the photo. ISO 800 should still look decent on your camera.

On the other hand. What do you consider a fast shutter speed? For a person running, a shutter of 1/400 or 1/500 should be plenty. Yes, 1/2000 is faster but you generally would not notice that if the person is only running (if they are running and trying to catch a football, shooting 1/1000 or a little faster is needed to free the thrown football, not the runner).

Having your images turn out dark tells us that you are NOT using your light meter.

If you need to shoot at 1/500th second and your widest aperture is f/5.6, then you need to increase the cameras ISO until you can get that shutter speed at f/5.6

Here is how you balance ISO, shutter speed and lens aperture

http://camerasim.com/camera-simulator/

Here are two samples of images captured at 1/500th second using the metered aperture. Both of these shots were taken with the camera in the manual mode. The correct exposure was determined and then the camera was shot at that shutter speed and lens aperture throughout the game.

Just remember, NO one learns how to control a camera overnight. It usually takes a few years and at least one class in photography

Spend some time here: http://www.nikondigitutor.com/eng/d5000/index.shtml Who better to teach you to use your Nikon D5000 than Nikon?

Then spend time here:
http://www.digital-photography-school.com/learning-exposure-in-digital-photography

If you are photographing runners outdoors on a sunny day there's only one reason why your exposure is wrong - you are shooting in Manual and aren't paying attention to the light meter. There's an old rule that was used in the days before cameras had built-in light meters - the "Sunny 16 Rule".It states: "On a sunny day set your aperture to f16 and your shutter speed to 1/ISO." So if its sunny out and we're using ISO 100 a "Sunny 16" exposure chart would look like this:

ISO 100
f16 @ 1/100 sec. "Sunny 16"
f11 @ 1/200 sec.
f8.0 @ 1/400 sec.
f5.6 @ 1/800 sec.
f4.0 @ 1/1600 sec.

Now if you are shooting in Manual and ignoring the camera's meter and happened to use f5.6 and a shutter speed of 1/1600 sec. Or faster then your pictures will be underexposed - dark. So if you are planning on shooting in Manual then you really need to learn to use the light meter.

So watch the video and learn to use your camera's light meter.

Of course, you could just do what I do and shoot in Aperture Priority and set the widest aperture your lens allows - f5.6 @ 135mm - and let the camera set the shutter speed. That should give you about 1/800 sec.using ISO 100.

NOTE: "Sunny 16" also disproves the need to use a high ISO when shooting action on a sunny day.