Say and tell mean the same thing but are used in different constructions.
Preposition to is never used after tell.
Commands and advice: Tell + object + infinitive – is used for commands and advice:
Remember the following phrases with say:
so you say
it goes without saying
Remember the following phrases with tell:
tell a story/stories
tell a joke/jokes
tell a lie/lies
tell the truth
tell the future (= to know what the future will bring)
tell the time (= know how to read a clock)
tell apart (=to be able to see the difference between two things/people)