How to take pictures manually the correct way?

I have been using a Nikon D3000 with the kit lens. So far I'm happy with my photos, but I want to know how to shoot manually, and I'm not talking about switching form A to M on the lens (I always shoot on M). I use the camera for photography/art class… I don't know the terminology too well so bare with me. There are settings on my camera 'wheel' that have M, A, S, P. I know A is for aperture, S is for shutter but that's it. I would also want to know what settings I should use when shooting low light (ISO Shutter speed and what not) I basically want to know how to change settings according to what I'm shooting (I manly shoot on the street)… Maybe there's a site out there that will help… I looked on DPS but I don't really know what to search.

The best source for you will be the camera's instruction manual. Other than that, try this site, they have tons of helpful information and charts.
www.cambridgeincolour.com
www.digital-photography-school.com

P mode lets you adjust some functions that are normally auto set by the camera in auto mode, but the camera works out aperture and shutter speed for you.

A mode lets you select your choice of aperture and leaves the camera to work out the right shutter speed, the other settings are changeable as above.

S mode lets you select your choice of shutter speed and the camera works out a useable aperture for you, other settings as above.

M mode means you have to set every setting yourself.

To get a picture you have to balance aperture, shutter speed and sensitivity (ISO) to get either a result or the result you intended.

For working out the settings for M mode start by looking up the Sunny 16 rule.

However for your chosen subject, street photography, why not let the camera do some of the work… To isolate a subject and blur the background you use a large aperture (a small f number) and to have the sharpest focus across the picture you pick a mid-range aperture (like f/8).So select A mode and dial in your smallest f number to pick out a subject and blur the background or dial in f/8 to have the sharpest overall picture you can get hand-held and get the camera to work out the shutter speed.

The sensivity is the ISO number. The more sensitive you set the camera the quicker it can take a picture, but the more sensitive it is the lower the picture quality. At night make the camera as sensitive as you find the resulting picture quality acceptable, maybe ISO 800 or the higher setting of 1600. Newer cameras give good results at over ISO 3200 or 6400.In the light keep the ISO as low as possible.

Edit - I should add there's a light meter in the camera as well if your dead set on using M mode which tells you if you're set to make a picture too dark or too light.

I was using manual mode for quite a while until I heard a very well respected professional say to le the camera do more of the work. On my Canon, I now use aperture priority mode and shutter priority mode the most.