What kind of memory can I upgrade to?

OK, so the genius at the bar said that I could upgrade my mac to 8 gigs of ram and here on ifixit.com it states only up to 6 gigs is possible. so as was searching for memory i found descriptions such as low energy, unbuffered, and unregistered. Is there a certain type of memory only apple and ifixit.com can provide? I was a pc girl only until recently and I used to just switch out my pc memory all the time. I rather sound like a newb and get the correct help before I do something bad to my macbook. Thanks in advance