The Exposure Triangle.
By balancing the three items on the triangle you will make sure you get the exposure and image you want.
F Stop or Aperture.
This is the most used and simplest tool for changing the depth of field, the amount of the image which is in focus.
The F stop if the size of the aperture that the lens allows light through. The f stop is shown as a fraction of the focal length of the camera so f2 the aperture diameter is half the focal length of the camera, whereas f20 is one twentieth the focal length of the camera so much smaller. Therefore, f2 is a small number but described as large and f20 is a larger number but described small as they referring to the diameter of the aperture.
When using the f stop to change depth of field you need to know large aperture has a shallow depth of field while the smaller ones have a wider depth of field.
The other effect the fstop has in the amount of light getting into the camera with the large ones giving more light and the small less. This means to get the correct exposure you need to balance a reduction in aperture with a decrease in the shutter speed and/or an increase in film speed.
Depth of field can also be controlled through
Focal Length of the Lens
The smaller the focal length the large the depth of field will be. The other affects of the focal length will be covered at another time.
Distance From the lens.
The closer the object is to the lens the smaller the depth of field will be.
The shutter speed indicates how long the sensor/film is open to light.
A fast shutter will freeze movement while a slower one will show movement as blur but as you will expect the faster the shutter speed the less light you will get.
ISO (Film Speed)
The ISO rating indicates the speed of the film or setting of the sensor. The lower the number the more light is required to get the correct exposure, but you will get higher definition in the image/less noise.
The amount of noise, grain, you get will depend on your make/model of camera or film.