Aperture asking for serial number
When and how did this start? After you made an update? Did you update from the App Store or download an installer from the support page?
How do I even find my serial number since I downloaded it from the ap store?
Jill, Aperture from the App Store does not have a serial number. If it is asking for one, your installation is buggy.
I'd recommend to reinstall Aperture from the AppStore.
Back up your computer, if you do not have a current backup.
Remove the Aperture application from the "Applications" folder to the Trash.
Sign into the AppStore and go to the "Purchased" tab. Download Aperture again and let it install.
Does it work now?
Also, in the future, how do I uninstall an ap?
That depend on the application. Applications, that are coming with an installer, usually also have an uninstaller. Use that to uninstall.
Other Applications, that you install by dragging them to the Applications folder, can be uninstalled by simply deleting them.
The documentation of each app should tell you, where the data reside, that also need to be uninstalled.