How to mirror MacBook Pro screen to Apple TV?
If your MBP is early 2011 or newer and there's an Apple TV on the same wifi network as you, you should see an icon that looks like a TV in the menu bar. Click it and you'll get a dropdown menu listing all the available Apple TVs (in case there's more than one). Click yours and you should be good to go!

If your MBP is older than early 2011, you'll need a third-party utility of some sort. If you're willing to shell out $15, I've heard great things about AirParrot.