The technical side of the photography breaks down into three large different areas.

1. Camera's exposure settings.
2. Lights.
3. Where you point your camera.

The 3rd one is fairly straight forward, and also the most difficult. It's often called the composition, but it's also a lot of other things, like travelling and getting up early and meeting interesting people (subjects), depending on what you do.

The 2nd one takes a while to master, but if what you have is just a camera, 'reading' the light (as in, looking at available lights) is the first thing you'll want to learn, as opposed to setting up your own lights. That can come later. The only way to do this is practice and reading and looking at other people's photos. Pick photos that you like, and just consider how the light would've been at the time. Where is it coming from and how strong and contrasty it is, and how many light sources there are, and so on.

The 1st one is what you're asking for. How to set your exposure correctly. This also used to be the technique that was expensive if you got things wrong (because film cost money). The good news is that the modern cameras are extremely good at getting this right. To the point where 99% of all photographs are being taken without the user even caring about it. Your phone knows how to do it, as your tiny pocket camera.

But if you're playing with a D90, you want to play with exposures. so here we go.

There are 4 things you need to know: the shutter speed, the aperture, the ISO, and the exposure compensation.

The Basics: The 'exposure' relates to the amount of light that your sensor (or film) will be exposed to the light. The amount if calculated by a simple formula: (the width of the lens) x (time the shutter was open). The first one is the aperture, the second the shutter speed.

This amount of light is registered by the sensor. The sensor can change its 'sensitivity' to the light, which is called the ISO.

Lastly, it's important to understand that the camera will measure light in a predictable, machine-like way, and will try to evenly expose the entire photograph. So, if the background is dark and your subject is bright, the camera will try to make the whole thing a bit bright so that it's even. This might not be what you want - you might want your background to stay dark, and subject normal. You tell your camera to make things a bit darker than normal, by dialling in an exposure compensation value.

It's also important to know that all 4 settings have other interesting side effects, other than simply modifying the amount of light. For example, a longer shutter speed will make small shakes in camera and motion of the subjects more visible. This should be straight forward to understand.

Smaller and larger apertures will affect what's called the depth of field, and this is a slightly complicated topic but there should be a lot of resources around to understand this.

ISO setting will affect the graininess, or the noise, of the photo. Sometimes this can be used as an effect, but most of the times it's considered a bad thing.

So what you end up with, is a 4-variable function the controls how your final image looks, even after having chosen the light and where to point your camera. A function full of subtle nuances.... and we haven't even started with the artsy fartsy bit.

So read about those things in the manual. Learn how to control all 4 of them without taking your eye off the view finder, (it's easy with D90) shoot RAW, and disregard all other functions of the camera, at least for now.