i think it should be tell. "To tell" is a prepositional phrase. If you took a prepositional phrase out of a sentence, it would still make sense. An example would be, " The dog ran to the water." To the water is the prepostional phrase. If you took it out (The dog ran.) It still makes sense. If you took "to tell" out, it would not make any sense at all. That's exactly why I chose tell instead of "to tell" . . .
i think u could use both because they both sound right with the sentence.
It is a verb pattern. I mean: This verb, in contexts like this one, is always followed by a certain preposition.