Focusing and exposure control are completely separate functions, although the lens aperture - the f/number - has a big effect on the depth of field (how much is sharp in the picture, in front of and behind the point focused on).
You need to understand what is happening when setting lens aperture and shutter speed, together with ISO, as all these settings have a big effect on how the image appears even though you can obtain the same correct exposure with a wide range of different combinations. This is the basic skill of photography.
For newcomers, I think using manual exposure control is potential dangerous. It is very easy for the light to change a lot even when shooting the same subject and the camera will keep track of that, usually with a good degree of accuracy. To say that you can correct things in post processing is a) only true to a small extend, and b) is bad practice that will lead to disaster sooner rather than later. There is no substitute for a bit of learning, checking the LCD image and knowing what the histogram is telling you.
I would stick to evalutative/matrix metering. It will always get you close to optimum exposure, often dead on. Then check the LCD/histogram and moderate it with +/- compensation if needs be. If you are confident that the exposure is correct and that the light will not change, then you can switch to manual which basically locks the camera settings so that nothing will change unless you alter it.