"do i go to a computer shop and buy ram?"
That's one way - or buy on line. Crucial is very good - they sell good quality RAM and have a fre configuration guide to enable you to select the correct type of RAM - and an xchange guarantee if they provide the wrong sort via their configurator.
You also need to ensure that there is a socket left in your system to fit more RAM.
If you need to know what's in your system right now - Everest click here is a free download to tell you all sorts of details about your system, including what RAM - sizes and tuypes - you have now.