This should be a lot of fun. The title describes itself, really. If you had the opportunity to say anything to a yourself from the past, what would you say. The previous you may or may not believe you, that completely depends on your personality from that time period.
I personally would say these things:
"Yes, it's scary at first because you don't get a physical copy, but you really, REALLY should be buying on Steam instead of spending 60 euros on dissapointing games."
"Borderlands/Bioshock Infinite/... will be one of the biggest dissapointments ever."
"You really should be spending more time studying more French and less studying how fast you can level up in WoW by taking a certain questroute."
"Those achievements/trophy-things you see? They are completely meaningless and worthless and you are being frustrated for NO GOOD REASON."
You can say anything to any past version of yourself, so go nuts!