Say and tell mean the same
thing but are used in different constructions.
Remember:
- We say something
 
- We say something to someone
 
- We say to someone something
 
- We tell someone something
 
Preposition to
is never used after tell. 
Examples:
- Mother said: “I am very happy.”
 
- Mother said (that) she was very
     happy.
 
- Mother said to me, “I am very
     happy.”
 
- Mother told me (that) she was
     happy.
 
Commands and advice: 
Tell someone (not) to do something
Examples:
- The teacher told us to stand up.
 
- She told me not to worry about
     the time.
 
Remember the
following phrases with say and tell:
 
so you say
it goes
without saying
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)
You can read more rules and take several quizzes here: