Check your motherboard manufacturing website for specs on your board....open the box and look at the memory slots,....two or three?
If you need to add memory to your system, you will have to shut it down and pull the cover off and look for
available slots on the motherboard. You will also need to "match" the type of memory your system already has installed.
If you have never done this before I would advise taking it to a pc shop.
Well, one thing is to go to the computer's user manual--this should be stated under the factory specifications.
If it's a major manufacturer (e.g. Dell, Gateway, HP, etc.), you can also find this information online with a simple Google search.
On most manuals, the vendor states how much memory is installed, and what (e.g. my system has 256MB installed, but can take up to 2GB total)
This web site http://www.crucial.com/systemscanner/ will tell you how much memory your computer has now and how much it will handle. It will tell you if you have any available slots and what type of memory is compatible.