The verbs tell and say have similar meanings, but are not exactly the same. Both of them mean "to communicate verbally with someone", but depending on context we use one or the other.
Tell is normally followed by a personal object. We usually use say without a personal object:
We told everybody that we wanted to eat.
We said that we wanted to eat.
He told me that he loved Dionte.
He said that he loved Dionte.
Tell has to be used in certain situations. In these situations tell can't be replaced with say.