Go to the web site for the company that makes the motherboard in your system (or if you have a system that was made by some company like Acer or Dell, or one like that, go to that companies web site). Find the list for Compatible Memory for that specific motherboard (or system). Then go to your favorite computer vendors site in a second tab, and find some memory that you like both the features and the price, and check to see if that memory is compatible in the list your motherboard company has.
When all the stars line up in the right order, and no lightning bolts come down on your head, you should then order the new memory. When you get it, unplug your computer from the wall. Let the power drain for 30 seconds or so before opening the case. I would also recommend using a grounding strap. remove the old memory from the system. Install the new memory.
We don't know what kind of system you have, so you will need to take it from there...