After upgrading your Mac to OS X Yosemite or later, the new Photos app gets installed automatically.
On first launch, Photos will auto-upgrade your default iPhoto library to its new library format. People with multiple iPhoto libraries must manually convert them to the new format by holding the Option (⌥) key when clicking the Photos icon in the Dock.
Upon completing migration, you'll have two sets of photo libraries on your machine: the original iPhoto libraries and their Photos counterpart.
In order to free up a significant amount of storage space on your Mac, you can safely delete any iPhoto library that has been migrated to the new Photos app.