MacBook Pro not screensharing with Apple tv?!?

Ok, so totally new to the Mac world, used PC up until about three months ago and now I just want to cry, give up and forget Mac and Apple exist in this world!

So I have a brand new MacBook Pro, bought in Aug 2012.

Got convinced by a sale woman to also buy the Apple tv a few weeks ago and I cannot get the stupid thing to work in screensharing mode. I want to be able to share my computer screen with the tv, like she showed me in the store but no such luck. I've tried to upgrade every software I have, in the tv and on the computer until I googled and found out that there is apparently this thing called Mountain Lion...new software that I supposedly need, is that true and do i seriously have to pay 20 bucks for the stupid thing too????? I mean apparently the software was released a month before I bought my computer so why didn't the salesperson tell me that I might need this, first the one that sold me the computer and then the one that sold me the tv?

Like I said, my first and last Mac, a seriously expensive lesson to be learned as i have given about 3000dollars for the stupid computer!

Ok, that was off topic, sorry! Main question, I need Mountain Lion to screen share right?

thanks!