The Anatomy of a DSLR
The main components that are needed to turn on a DSLR camera and properly use it are the camera body, lens, a battery and a memory card. Some of the more basic components of a DSLR include the viewfinder, aperture, shutter, and the digital sensor.
The viewfinder is what the photographer looks through to see the image that the camera is capturing. The aperture is the hole located inside the lens that opens and closes to allow a certain amount of light through the lens. The shutter button is what allows the photo to be taken and it activates metering and auto focus when it's halfway pressed. The sensor is a mechanism that is made of light sensitive materials that record an image. It's the mechanism that converts light into electrical signals that turn into visual data.
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DSLR Camera Shooting Modes
The four shooting (or exposure) modes that are found on a DSLR are Programmed Exposure (P), Shutter-Priority (TV or S), Aperture-Priority (AV or A) and Manual Exposure (M). Programmed Exposure is fully automatic while Shutter-Priority is when the shutter speed is determined by the photographer and the aperture is adjusted by the camera. Aperture-Priority is when the aperture is determined by the photographer while the camera adjusts the shutter speed. Manual Exposure is when the photographer has complete control over how the settings are adjusted for exposure.
How to take photos in Manual
To properly expose an image in Manual Exposure, it requires an understanding of the Exposure Triangle. The three elements that are a part of the Exposure Triangle are the shutter speed, aperture and ISO.
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To apply the Exposure Triangle to taking a photo, a tool that is very helpful for measuring the amount of light included in an image is called the light meter. Every DSLR includes a light meter that automatically measures the amount of light in the frame and determines the proper exposure. To avoid photos coming out to be over-exposed or under-exposed, aim to have the line resting on the center of the meter.
Refer to this link for more on Understanding Metering.