It sounds like you're looking at its purchase page, not an iPhoto update.
A new mac computer comes with iPhoto, Garage Band and iMovie. They are usually not already installed. To get them, click the apple in the upper left, click App Store on the menu. When that opens up, click on the Store menu at the top of your screen. Sign in with your apple id. Back in the app store window, click on the purchases tab. You should see the mentioned applications waiting for you to accept them. When you do, they should start downloading.
If they are already installed, they'll be in your /Applications folder. If you want to check for updates for them, click the apple, then software update.
If the computer is used or opened box, you can run into issues when someone else has already accept those applications, but it sounds like you probably have a new one...