The new macOS has replaced iPhoto with an app called Photos.
Your existing iPhoto library will need to be upgraded before you can use it with Photos.
Step 1: Browse to the Pictures folder in your Home folder (see below).
Step 2: Double click the iPhoto library. This will now be upgraded, the amount of time this takes will depend on how many photos you currently have in your iPhoto library.
Once completed, you will now notice a Photo library alongside the old iPhoto library. Once you are happy that all of your photos have transferred correctly, the iPhoto library can be deleted.