Inside your camera, there is a light meter. It measure reflected ambient light and guess the correct exposure or exposure value (EV) for you. And use the settings, or say a combination of shutter speed, aperture and ISO value, to meet the EV.
So, a lot of time, you can simple point at the scene and click. Of course, there are exceptions.
If you do not want to use the in camera meter, or there are some artificial lights added to the scene, or you have a camera that do not have in-camera light meter, you can use a external light meter to measure the light and set the settings accordingly.
Or in general, you can use the EV table to figure it out. Of course, it is NOT going to be exactly the same, but close.
Go to the link listed below.
Exposure value - Wikipedia, the free encyclopedia
Scroll down to Table1, you will see the EV table.
Scroll down to Table2, you will see a EV for various conditions.
i.e.
In table2, heavy over cast scene, EV is 12 with ISO100.
Now go back to table1, if you like to shoot at f/5.6, according to the table, the shutter speed is going to be 1/125.
So in your camera, you can set ISO100, aperture f/5.6 and shutter speed of 1/125. If you like to shoot at f/2.8, then shutter speed need to change to 1/500. (2 stops faster)