Aperture size is usually adjusted in f-numbers or f-stops. i.e. those little numbers engraved on the lens like f22 (f/22),16 (f/16), f/11, f/8.0, f/5.6, f/4.0, f/2.8, f/2.0, f/1.8 etc. f/16 will let in 1X the amount of light than a diaphragm opening of f/22 and so forth; while on the other hand, an aperture of f/4.0 will let in 1X lesser than that of f/2.8 etc.
Always remember that large aperture = small f-stop = shallow depth of field (f 1.8, f2.8, f4, f5.6) whereas small aperture = large f-stop = longer depth of field (f 11, f22, f32)
To blur the background use a large aperture (small f-stop) and get close to your object and zoom in. For infinite depth of field use a very small aperture (large f-stop) and step further away from your subject or use a wide angle lens.
f/8 to f/11 are often the sharpest aperture f-stops and offer the greatest contrast in exposures.
f/1.2 to f/5.6 (depending on your lens) produce a blown away background and thus are great for outdoor portraits.