Use a command for:
Instructions
Write your name at the top of the page.
Don't write anything below this line.
Orders
Clean your room now!
Don't turn on the T.V.
Requests
Please call me if you have any problems.
Please don't hesitate to ask questions.
Warnings
Watch out! Be careful! Don't slip!
Use Let's for invitations to do something with you.
Let's go to that new restaurant.