Use "tell" when information is given. Don't forget that you always have to include someone to tell to. "I told him about the party tomorrow night." Use "say" when someone is simply speaking. A person can be alone, and still say things, without a listener. "Talk" suggests an interaction, so you need two or more people. It's a two-way process (unlike "tell") and what is said does not have to be important. "I talked with my brother, and told him he was an idiot. He said something, but I had already left the room."

With 'tell' we usually say who is told. You could say there is a personal object, so: 'Can you tell me what's happened?' We say, 'me'. With 'say' we don't usually say who is told. So you might say 'please say each word clearly and distinctly'. And if we do say who is told, we use the word 'to', so: 'He said goodbye to me as if we would never see one another again'. Now... There's another limit on the usage of 'tell'. We only use 'tell' to mean instruct or inform. 'I told him to wait for me on the platform'... that's an instruction. "My father used to tell me wonderful stories" - informing me. 'Say' can be used for any kind of talking. So here are three sentences where you could not use 'tell': She said 'Where have you been?' So I said what a good idea. Maureen said 'What's the matter?' We use 'tell' without a personal object in a few expressions, that are kind of fixed expressions like tell the truth, tell the time and tell the difference. And we use 'say' before words like a word, a name, or a sentence. An example would be: 'Don't say a word.' PS. i hope this will help :)

I'll give you some examples from Russian version of Esquire magazine. "Tell" is used in 3 cases: 1) When you suppose to get the full explanation of topic, long story. Example: When little child see porn for the first time he asks his mother "Tell me everything you know about this, Mom!" 2) "Say" is more often used with direct speech, "tell" - with indirect. Example: "Tell John to bring my gun back, I'm gonna kill my English language teacher!!!" 3) If words "tell" and "say" can be interchanged by meaning, "tell" always is used without "to". Example: When you stand near dead body of your friend who was bitten by tiger you may say "He did tell (say to) me he had a bad feeling about pulling out that tiger's teeth..." To learn when "say" is used first review previous 3 items and then learn by heart next: "To tell: a story,a lie, a joke, a secret, the truth, the time" BUT "to say: yes or no, a few words, to say something" So next time when your teacher would ask you "Say something!" you can boldly say "SOMETHING!"