My kitchen floor by the back door has begun to dry rot. I would like to replace it so that I can lay some new tile. I have no idea the amount of work it would take to do it. Can anybody help me?

Find the joist the floor sits on and cut out that bad section.

When you cut it set saw depth for the sub floor thickness and cut in the center of the original joist.

You will need to add a 2by scab between joist to old and new floor sit on it and can be nailed there because you loose the tongue and grove on a floor repair.

Look for water damage that's usually the cause for rotting at one time or another. The back door threshold would be good to check and the frame caulked on the outside.
Use liquid nail or a caulk to apply over joist before laying in new sub floor to keep squeaks out. Also if you have an old floor use 3/4 for sub floor and the Luan or 3/8 floor to flush with original floor.