I don't have the slightest idea what my computer is running on..how do I find out?

My daughter gave me an MacBook Air in April 2012. I don't haue the slightest idea what it is running on. How can I find that out? All I know is that when I went to order pictures the computer said should update to iPhoot'11 update to 9.2.3 or later before placing an order. How can I find out what system my computer is runing on?