Get memory that is the right speed for your computer so that it can use the memory without tripping. Somewhere, it should say how fast the RAM is. Consult your motherboard's manual to see what kinds of memory it can handle. In general, you don't want to get memory that is any slower than the memory currently on your computer. Make sure you don't exceed the amount of memory your motherboard can handle.
Different boards have different limits.
Again, consult your board's manual for this information. This is how to put RAM in. Keep in mind that some computers have oddball chip sizes. If the instructions are Greek to you, head to the shop. Turn off the computer, unplug it, and take off the case.