1) He will tell you what you do tomorrow.
2) He will tell you what you will do tomorrow.
3) He will tell you what you shall do tomorrow.
In which case:
a) He is going to make a prediction about what you will do tomorrow (maybe he is a fortune-teller)
and in which case:
b) He will tell you what to do tomorrow. (He is probably your superior and is going to give you an order)
This is the place to post questions and discussions on usage and style. The members of the Wordwizard Clubhouse will also often be able to help you to formulate that difficult letter.
End of topic.