My dishwasher is overflowing with a lot of water (not flowing into garbage disposal?

If you have an Air-Gap installed see the image below, you have the line marked Marked red may be blocked remove the line and clean, see if that will help,
Also If you a new garbage disposal installed may be the knockout plug is not removed remove that, Good luck.

John

In addition to John's advice - make sure the short hose, that connects Air Gap and GD ( in red on John's drawing ) is not kinked. Also, make sure there isn't rust or old food blocking DW drain inlet on the inside of the Garbage Disposer

Milo