After pressing Get Started in the blue box as seen above, you have two different options. If you are brand new to photo organizing on a Mac and have never used iPhoto or Aperture before, (or if you don’t want to migrate your iPhoto or Aperture library to Photos), you have the option to import pictures from your digital camera or SD card, drag files directly into Photos, import pictures from the File menu or turn on iCloud Photo Library under preferences.
If you were previously using iPhoto or Aperture, after clicking Getting Started, it detects your iPhoto and Aperture libraries. Select which library you want to use with Photos.
Once you select the library, it presents the option to set up iCloud Photo Library, Apple’s iCloud-based photo and video syncing and storage service. This will allow you to sync all the Photos from your Mac and iOS device to each of your other devices, keeping your edits and albums in sync. It then starts to prepare your library to transition to Photos. If you are not sure about putting all of your pictures into iCloud Photo Library, you can always set that up later under Photos’s Preferences from the menu bar; it’s optional and never required unless you want automatic, cloud-based syncing.
Your old photo libraries will still stay on your Mac if you decide you want to use iPhoto or Aperture for doing something. One might still want to use Aperture because it is a lot more advanced with support for plug-ins and more for professional photographers, with powerful editing tools and built-in support to use external editors. However, if you make any changes in those libraries, they will not automatically sync over to your new Photo library.
Most of the data from iPhoto and Aperture will transfer to the Photos app. For example, Faces is still the same. Also, if you were using star ratings, flags and color labels in iPhoto and Aperture, they are still around in Photos, but they are now keywords and assigned to the photos. You’ll notice your Events from your previous photo library is organized alongside albums in the new Photos app preserving your organization.
That is how you move your iPhoto or Aperture library to the new Photos app. If you haven’t done so, make sure you set up iCloud Photo Library on your iOS device. Let us know if you have moved your library from iPhoto or Aperture to Photos and what your impressions are in the comments!