Why do my pictures look very dark on my digital camera?

I just got a Nikon D90 and I admit that I'm still trying to figure it out.
One thing that I find a little confusing is why when I try to take a picture the image on the screen looks fine but after I click the button it looks very dark.
I don't know which settings to adjust so the picture looks the way it's supposed to.

This will help you in all matters.

"I just got a Nikon D90 and I admit that I'm still trying to figure it out."

I bet your d90 came with a user manual.
May I suggest that you spend some time reading it, then you will know All about your camera.

If you had any experience from taking a class or two in photography, you would be able to see that those dark images are due to under exposure

My guess that you did not even check the light meter in your cameras viewfinder either.

Fine camera like the D90 are NOT push button cameras, but require the user to have at least a basic understanding of exposure and using other camera functions.

You can download the user manual and refer to it as you learn how to use your camera.

Taking a class in photography is a much better way to learn the necessary techniques to operate a camera than the "try and fail" method you seem to be using

Just put it on one of the Auto modes, until you have figured out how to work your camera. If the user manual isn't doing it for you, there are third party books that can take you through the basics like this: http://www.amazon.com/...zon.com/s/
and this: http://www.focalpress.com/content.aspx?id=4148

Information is a two way thing,

You only state you are using a D90 and the photos are dark. What setting are you using? Most likely not the Auto. You don't say what kind of photograph your trying to take, like a portrait or a bowl of fruit on a table, a dog in the park. Subject location and light source help determine proper settings.

You need to learn:
Apature
Shutter Speed
White Balance
ISO

Add more info and a better answer can be given.

And those downing the answers telling a person to read the manual, or taking a class. Why? The manual should be kept in your camera bag for reference, And classes can be found for any level of photographer, and face to face help is priceless.