Welcome to the Apple Support Communities
First, open > About this Mac, and check Version. If it is:
- 10.7 or 10.8: You have FaceTime on the Dock, so open FaceTime.
- 10.6: First, open > Software Update and install the most recent version. Then, open App Store and purchase FaceTime.
- Older Mac OS X versions: Check if your Mac is compatible with Snow Leopard > http://support.apple.com/kb/SP575 Then, purchase Snow Leopard > http://store.apple.com/us/product/MC573/mac-os-x-106-snow-leopard