You shouldn't be. All your photos should still be in the iPhoto Library file which should be in your Pictures Folder.
When you run Photos for the first time, it should import all your pictures from the iPhoto Library file, and they should appear in Photos. Has this happened?
If not, and you still have the iPhoto Library file, you can access it and import from there. However, it is not a standard folder that you can open by double clicking. It is a Package. To open it, Right/Ctrl/Two Finger click on it. Inside there will be a number of folders. All your original photos will be, unsurprisingly, in the Originals Folder, and any that you have edited/modified will be in the Modified Folder.
This tells you how to import your iPhoto Library into Photos, although I haven't tried it.
http://osxdaily.com/2015/04/17/move-iphoto-library-to-photos-mac/