My toilet waste is backing up into the shower

You have a blockage in your waste line. Run a snake down shower drain and out about 10 feet. If this does not clear problem you may have to run it out more.Run hot water after it clears for about 2 minutes.

Chuck

You can also find your cleanout and snake out to your main.

You may also lift the toilet and insert the snake through the toilet opening in the floor. Moreover, you may also snake your toilet pipe through vent from the roof ...

Milo

I agree with the experts above please follow theirs instructions, good luck.

John