Get a small screwdriver and open up the panels on the underside until you find the ram slots to make sure there is space. In some instances the manufacturers use smaller memory modules to save money, and fill all the slots. Or in some machines (I know some Acer Ferrari's) you need to remove the keyboard to add more ram.
Once you've established there is a slot free, get on Crucial's website and use their configurator to find a match.