say + [something]
say + [something] + to + [person]
tell + [person] + [something]
Both verbs are about speaking, and both are irregular: say – said – said, tell – told – told. The difference is in what comes after them.
Tell is followed by the person who hears: tell me, tell Yuki, tell the teacher. Then comes the information.
Say is followed by the words or the message. If you want to name the person, use to after the words.
Say is also the verb for quoting someone's exact words.
Some phrases always take tell, even without a person:
For an order or a request, use tell + person + to + verb.