Adding RAM to your computer mac or windows whatever, is the best thing you can do to make it faster, original RAM, especially from apple, is very expensive I have found a website where the quality is the same but 6 times cheaper, some computer have the RAM welded to their motherboard, for instance MacBook Air it's possible to change it but hard etc.. the best way to do it it's the following :
Going to this website : crucial.com try to stay on the US version at least to figure out what ram you need.[US version is more complete]
Then go on top, on memory, and download the system scanner.
Run the system scanner just downloaded and boom it will come the ram that your computer can support and need
If you are based in the US just buy it otherwise keep note of the ram that came out, take a screenshot, get exactly the same one there's tons of RAM don't make a stupid mistake then buy it on your local store based in Europe, Pacific or whatever.
When you get the RAM shipped home within few days, it's going to be easy to install it. There's plenty of videos on YouTube explaining how to do it and it's pretty much for all the computers the same thing, just be very careful because you can seriously harm your computer forever. You're not playing with a Barbie.