First thing is to clearly identify the brand or model of your computer. Usually the brand is obvious - such as Dell or HP, and the model number will usually be printed on the case somewhere, or on the base if you have a laptop. If you have a hand built computer then you will need to identify the brand and model of the motherboard inside the case. If you are not sure of this information then do a search on "cpuid" or "belarc system advisor". Either of these programs can be quickly downloaded and installed to give you specifics about your computer. (note these programs only work on windows computers)
Work out how much memory you have installed, and how many modules. You can find this out by either doing a physical inspection (that is, take off the case, or unscrew the memory hatch on the base if you have a laptop), or by using the cpuid software.
Now you need to find out the ram upgrade options for your computer. There are a few sites on the internet which provide specific information about thousands of different model computer systems. RamCity is one of these, or you can check the Kingston site. This information includes the maximum possible installed memory your computer can handle, as well as the number of upgrade slots.
Now that you are armed with information on the right upgrades, and how much they cost - it's just a matter of deciding how much memory or RAM you want to purchase. If you are running Windows XP, Linux and MacOS, I recommend somewhere between 1 and 2GB of RAM. For Vista, this rises to 2GB to 4GB. If you are a serious gamer, then I recommend buying as much as you can afford.
There are Ebay sellers who sell generic memory upgrades, but beware as these may not always work in your computer. For maximum piece of mind - I recommend buying an upgrade that is specific for your computer - like those made by Kingston.
Be sure to buy from a seller who provides a guarantee that you can return the memory for refund or replacement if it doesn't work. Better still - buy from a seller who specialises in memory upgrades and you can be more confident that the RAM you receive will work the first time.
Now it's time to install your memory upgrade. If you are not sure of where to begin, then your user manual is a good place to start. If you don't have one, then there are a few sites on the internet with installation guides to suit most computers and laptops.
This guide is authored by Rod Bland, the owner of RamCity - a specialist computer memory upgrade supplier.