iPhoto

iPhoto was discontinued and replaced by "Photos." Hold down the option key on your Mac while opening the Photos app and you will have the option to select your iPhoto library. Photos will convert it and open it in the Photos app.

iPhoto was discontinued 6 years ago. It is no longer developed or sold. It was replaced by the more powerful (and free) Photos.app that comes with the OS. You can migrate to that app very simply:

Hold down the option (or alt) key and launch Photos. From the resulting menu select 'Choose Library' and point at your iPhoto Library. It will do the rest.

If you prefer not to use Photos there are several 3rd party apps that you can use. Which to choose depends on the kind of photographer you are and your budget.