Hi @OMom2019
In addition to the brilliant resources and tools that @Miles B. has pointed you to. I would suggest you have a look at this video from Remote Summit on how Arm approached the move for 10,000 users to Cloud. This should give you some great pointers on how to think of the business strategy behind a migration to make sure you are not just transferring problems from one solution to another.
https://www.youtube.com/embed/qjmNTWArrgw
(I will declare a personal interest in this one as I was involved in this migration and the presentation)
Hope this helps you have a successful migration.
Phill