This will depend on the age of your computer and if you have any available RAM slots on your motherboard. If you have an older computer it may not be easy to find the right kind of RAM for your computer since it’s not all the same and will depend on the type of motherboard you have. You can take out a RAM chip/DIMM and see if it has a sticker on it saying something like DDR3 PC2800 to give you an idea of what kind of RAM you have.
If all your RAM slots are full then the only way you will be able to upgrade will be by taking out some of the smaller RAM DIMMs and replacing them with larger capacity ones. RAM is pretty cheap and easy to replace so it’s a wise investment if your computer does not have enough.
For Windows Vista and up you should have a minimum of 3 GB with 6 to 8 GB being the ideal amount.