how do i get iPhoto to work on catalina update

Here are some user tips from Photos for Mac  User Tips that will help you in your transition from iPhoto to Photos:

Notes on Migrating an iPhoto Library from iPhoto to Photos for Mac - read first

External Editors in Photos Are Here

Photo Editing Extensions for Photos for Mac

Post 9/30/2018 Printing Options for Photos - Apple Community

Ordering Books and Calendars on a Mac that cannot be updated past El Capitan 

Use Project Extensions to order print products in Photos after September 30, 2018 - Apple Support

Create and order print products in Photos with Project Extensions - Apple Support

Photos replaced iPhoto several years ago. To use iPhotos with your media: Launch Photos with the option key held down. From the resulting dialog choose your iPhoto Library. Photos will make a new Library with your iPhoto Media. Your iPhoto Library will not be changed.