How do I clean grease off kitchen cabinets to prepare for painting?

You try washing them in some very hot water with solution of dawn dish soapy water and vinegar water wash and rinse well keep I mind you be changing water often

I use Krud Kutter which is an alternative to a TSP cleaner, it gets rid of grease safely. https://amzn.to/2UE8gb3

Vinegar typically works great for grease, but if it's really bad, you might need to use TSP. Read the directions and take the right precautions when using it.

TSP is what I always use before a paint project.

Use TSP in hot water and wear gloves!