You're going to need to learn about "exposure".
I think you mentioned in another thread that you want to do close-up photography to shoot models. I'll answer based on that assumption.
The camera collects light and records the information. But the camera lens can constrain the size of the opening (the "aperture") through which light can pass by dilated or constricting aperture blades.
If these blades are wide open, the camera collects more light in a given moment. If they are constricted to a tiny opening then less light flows through. The camera also controls how long the shutter should remain open. So if it's collecting a lot of light, it can remain open only a short time... but with very little light, it can remain open for a longer time.
As long as the sensor gets the right amount of light (not so much that the pictures are blown out and too bright, but not so little that they're too dark) you should get useable picture.
But it turns out the "look" of the image will actually change depending on which settings you use.
You can vary shutter speed when shooting moving objects to either freeze them in place... or to deliberately create a sense of motion blur. Since your models are not in motion, this setting problem isn't going to change much for you.
However, the aperture setting changes focus area by affecting something called the "depth of field". This is the range of distances at which things will seem to be in acceptable focus. In other words, if something is 5' away and I focus on it, with a very shallow depth of field, then closer objects (say... 4' away or closer) will seem blurry and also farther objects (say, 6' away or farther) will ALSO seem blurry. But if I use a broad depth of field, then objects from, say... 3-10' away will all seem more or less in acceptable focus. You control this by changing the aperture size. Large openings produce a shallow depth of field and narrow openings produce a broad depth of field.
Since you are shooting "close up" images but want your whole subject to appear to be focused, I'd suggesting:
1) Use "Aperture Priority" mode - this means you'll rotate the mode dial to the "Av" position (Av = Aperture value)
In this mode, you select the aperture value that you want (I'll suggest you use f/22) and the computer in the camara will meter the light and automatically select the correct shutter speed for you. So you wont need to worry about getting all the exposure settings right... you only need to set the Aperture value (which is why it's called "Av" mode.)
2) You will need a tripod. When you dial the aperture down to f/22, not much light can pass through the lens (but you get a very broad depth of field - meaning quite a lot will be in good focus). However, since not much light is passing through the lens, the shutter will need to remain open much longer. Normally you can hand-hold the camera while shooting. But when the shutter needs to remain open longer, it is not possible to hand-hold the camera steady enough. So you need a way to make sure the camera cannot move during the exposure and the best way to do that is to use a tripod.
There is one more thing... sometimes the act of just touching the shutter button to take the shot can create enough vibration to blur the shot. So photographers will often use a remote shutter release. If you don't have a remote shutter release you could buy one... but the other trick to use the camera's built-in delay timer to take the shot. E.g. if the camera uses a delay (most models support either a 2 second or 10 second delay) then it gives you enough time for the vibrations to settle and then it takes the shot.
You might want to pick up a copy of the book "Understanding Exposure" by Bryan Peterson. It's a very good primary on learning to use your camera in manual mode to control exposure for more creative results then you could get by just leaving the camera in automatic mode.
Also, here's a pretty good video that simplifies the basics of exposure:
Tim Campbell
5D III, 5D IV, 60Da
