Both are verb associated to communication but both are used quite differently. Look at some of the examples of how to use these words.
say something/say something to somebody/say as a reporting verb.
- Ask him to say something in English.
- She didn’t say anything to him about the party
- She said that she liked it very much.
Say in impersonal passive structures
- It is said that many lost their lives in WWII
Say + what/how/when/why
- He didn’t say why he had come.
tell somebody something / tell somebody to do something
- She told him a bedtime story
- They told me to get the job done as soon as possible.
tell somebody about something/tell a joke or a story
- The company manager told them about the strike.
- Mike told us this really funny joke.
We don’t use say to somebody.
- She said to me that I couldn’t go. She told me that I couldn’t go.
It is much more common to use ‘tell’ for orders
- The fireman told us to be careful.