Why do my Nikon D3000 Pictures Comes Up Dark in M mode?

Basically I've been messing around with the settings here and there trying to figure out how to do long exposure and such, but problem is is that When I switch my camera to M mode and take a picture it comes up black. Why is this? D

Are you setting the ISO, shutter speed and aperture according to the meter? Try this: set the ISO to 1600, and then take a meter reading. Set the shutter and aperture according to the meter. Take the photo. It should come out.

Read, study the camera manual for info on how to manually adjust your settings.

The aperture controls the amount of light passing thought the lens and the shutter controls the amount of time the light is allowed to strike the sensor, both work together to control proper exposure, you need to read the cameras manual to understand this and it's normal to have to read it many times over

That's manual mode… You need to adjust your shutter speed, aperture, and ISO to expose correctly. There's an in-built lightmeter in DSLRs… You need to look inside the viewfinder and set the meter on the centre line for very basic correct exposure. In the M mode you have total control over exposure.

Take a photo in "P" mode. Look at the LCD display after you are done and see what ISO, shutter speed, and aperture the camera chose. Move to "M" mode and set the same settings manually. Shoot the picture. If you want it a little darker or lighter, just move the shutter speed or aperture up or down ONE CLICK AT THE TIME until you get what you want.

This is NOT how an experienced photog would do it, but since you don't want to read the manual, this is one way to learn about camera settings.

Your underexposing, because you didn't use the meter to check the exposure. I think the build in guide more will help you understand exposure, reciprocity and how the metering works.

In matrix metering try to get the meter needle to be in the middle (at 0EV)