Or you can ask someone who already knows. Through the Internet, even!
Anyway, all you have to do to upgrade a computer is check which motherboard you own, go to the mobo's company site, check which kind of processors, graphics cards and RAM it supports, and get the required components. And if you are not sure you can ask someone. Really, computer hardware savvy aren't exactly in short supply these days. Someone may not know much about the subject, but I'm sure everyone knows one or two people who do have the necessary knowledge.