A warm welcome to the Forums.
iPhoto has been replaced by Photos. Well, when I say replaced, technically the iPhoto app should still be present in your Applications Folder, but almost certainly Photos will now be in your Dock. Don't worry, you have not lost anything. The iPhoto library is still there, but Apple's plan and advice is to import your iPhoto images into the new Photos app. This doesn't involve you in any work. If you open Photos, it will ask you to import the library and, if you agree, it does it for you.
You can keep iPhoto if that's what you wish, but it is clear that support for it (and Aperture) will cease sometime soon.
Now, the point is Photo Stream will now ONLY work with Photos. So, if you want to retain that facility, you should open Photos and let it import from the old iPhoto. When that is done, open Photos Preferences > iCloud and turn on whichever services you want - these include "iCloud Photo Library", if you want it, and "My Photo Stream".
Hope that helps. Please post back if not helped or if you need more info etc.
Ian