Which sentence is grammatically correct:
I will tell your consultant.
I will tell to your consultant.
This is correct:
The verb tell is transitive and takes a direct object that indicates the person that will be spoken to. It is incorrect to add the preposition to.
Edit: Actually, it's a little more complicated than that. Let me explain.
The verb tell is actually ditransitive, meaning that it can take both a direct object and an indirect object:
Here the bolded you is the "indirect object", and it indicates the person that will hear the story. The italicized a story is the "direct object", and it indicates what will be told to the person. When tell is used ditransitively, you can optionally take the indirect object and move it to a prepositional phrase with to:
(Almost all ditransitive verbs in English allow you to move the indirect object to a prepositional phrase with to.)
However, tell can also be used with just a single argument object, and in that case you never add the preposition to.
This is somewhat unusual, since the object of tell in this case can be either the indirect object or the direct object. Most ditransitive verbs don't work this way:
The verb "tell" has both transitive and intransitive usages. The transitive form -- a form that conveys the action of the verb to an object -- is direct. You can tell your consultant a story, or you can tell a story to your consultant. But in either case the action must relate directly to an object or person.
"Telling to someone" therefore sounds like the object is missing.