Just go outside on a sunny day and start shooting whatever's nearby. And aren't I glad that this is a camera thread to give context to that sentence...
Anyway, you want to understand the relationship of three things, represented with different numbers: Shutter Speed, Aperture, and ISO. Changing either one of them will affect how bright/dark your images are, AS WELL AS one other "side effect" unique to each choice. All three affect each other and you will eternally balance with them.
-Your shutter speed seems intuitively obvious: How long your shutter is open, or rather, how long your sensor is exposed to light. Measured in fractions of a second, or whole seconds or even minutes for really long exposures, but for walking around holding the thing in your hands, you're going to want numbers like 1/50 or 1/100, or even 1/1000 or 1/2000 in broad daylight. SIDE EFFECT: Shutter speed is also what you use to freeze motion. Something moving very fast will still be blurry even at 1/100th of a second. At 1/4000th, however, you might be able to freeze the subject. But getting that high of a shutter speed cuts down the amount of light you have, which leads us to...
-Aperture: How big a "hole" your sensor is being exposed to. Think of a water faucet; on low it takes forever to fill up a cup, but open it up a little and suddenly your cup runneth over in no time. Sometimes water pressure can be so strong it'll blast in your glass and splash out all over the place. With light, that last effect leads to "overexposure", when it's so bright that all you get is stark glaring whiteness, all detail of your subject gone. So you reduce the size of the hole that light (water) is going through, so it doesn't overload the sensor (cup). They call this "stopping down". One full "stop" refers to a halving or doubling of light; the water faucet becomes half or twice as strong, respectively. So the next number you have to worry about is your "f-stop", the number system that describes how "open" or "closed" your lens is. f/11 is a very small hole, and you'll probably only use this in broad daylight. Lots of zooms have a widest f-stop of 3.5; more expensive zooms will go to f/2.8; some prime lenses (lenses that don't zoom) go down to very wide f-stops like 1.8 or 1.4. Very small numbers (wide apertures) mean lots of light per unit of time: at 1/100th of a second, f/2.8 will be "twice as bright" as f/4.
SIDE EFFECT: This is a biggie, and it's Depth Of Field. This refers to how wide an area, perpendicular to the front of the lens. Narrow depth of field
(DoF) has everything on one narrow plane in focus, while everything behind it or in front of it is blurry; this is a product of wide apertures (small f-numbers, like f/1.4 or f/2 or whatever) in most shooting situations. Deep DoF
, or wide, or thick, or... whatever... has a very large plane of focus, at very narrow apertures (big f-numbers, like f/8, 11, 16). Although I confess, in that last photo I cheated and used a very wide-angle lens; I'll give you a note about focal length below.
-ISO is just how sensitive your camera is to light. This is a very simple one: Noise. Low ISO's mean low noise, and low sensitivity (you'll need wider apertures or longer shutter speeds unless you're in broad daylight). Noise is random data of wrongly-colored pixels. When there's less light, the camera has to "guess" more, and makes more errors based on random spikes of information in the sensor. The technical details are fascinating, but are also minutiae when discussing shooting. What you really need to understand is that the clearest, sharpest, best colored images will come at the lower ISOs. Higher ISOs make shooting in very low light possible. The D90, in my opinion, looks very good even at ISO 1600 or even, depending on the subject, 3200, but try the different settings and see what you like. You may prefer to keep it under ISO 800. I know I try to keep it at ISO 200 as much as possible.
ISO is sort of a buffer that gives you wiggle room between balancing your aperture and shutter speeds. If you really want a deep depth of field, need a high shutter speed, you have no choice but to raise the ISO (or add more light; open a window, turn on more lights, use a flash or multiple flashes... photography can get expensive really quick).
-Focal length: How "zoomed in" or "zoomed out" you are, in simplest terms. Small numbers mean a wide angle of view (how much of what's in front of you will you see) while big numbers mean a narrow angle of view, to frame really tightly on distant objects. Roughly "in the middle" (thematically, not numerically) is the "normal" range. This corresponding number is commonly viewed to be hovering around 50mm in good ol' 35 mm film parlance; for a DX camera like your D90, which has a smaller sensor, "normal" is right around 35mm. Numbers smaller than 35 are "wide", and numbers higher go into "telephoto" range.
Wider lenses, all else being equal, will also have deeper depth of field. A 15mm focal length at f/4 will have a greater range of objects in focus than a 100mm focal length at the same aperture.
Shutter speed doesn't affect depth of field, but it will affect how blurry (or otherwise) your pictures are. At 1/30th of a second, even casual movement will make blur. At 1/5th of a second, your hand holding the camera will be so jittery that the entire image will blur, unless you have a very good Vibration Reduction system in your lens.
Hmm. I wound up writing way more than I thought I would, and I'm kinda running outta steam. I know I'm only beginning to scratch the surface. Sorry for the abrupt conclusion; congratulations on the new toy! Play around with it, go take some pictures, and realize that understanding the numbers and the tools are only to facilitate your own vision. Composition will ultimately be the most important thing for you to work on, and you'll be going nuts with it once you get over what I consider to be a not too steep learning curve.