There is a girl that is my friend on facebook, but, i dont know who she is. I know that she is on my facebook because one day i was at a friends house and she called him, but he was on the shower and told me to talk to her.
I talked to her and she add me on facebook, but it was 3 years ago, and i dont know how to talk to her now cuz it may look strange it could be awkward and my intentions could be really clear, i know that if i make any mistake, it could ruin my chances. I'm not afraid to try anything, i'm looking for advice for the best way to aproach via internet.

I'm a 16 years old Brazilian so sorry for any poor english.

If you contact her after this long, she will more than likely not remember you, and it'll put you in a position where she seemed to be too important.

Then again, you have nothing to lose, so I would just say "Hey we met x number of years ago and I wanted to catch up". She's also more than likely a young girl, so she isn't experienced enough to realize this.

You could start a conversation with something like "Hey.. Who are you? I don't remember adding you to facebook? Have you studied with me or something?"

Then if she doesn't remember, you both start asking questions about each other. Then you're on your own =)