In general it is hard to say which word we must use: “tell, talk, say, or speak”.
It´s almost always a big question, full of doubts, and it´s completely understandable because all these words are referring to the same sort of activity.
Let´s see the differences
Generally, “say” is used to introduce a mention, tell what someone said:
Paul said: I don’t like lies (Paul disse: eu não gosto de mentiras)
I say the truth all the time ( Eu digo a verdade o tempo todo)
When we say for who we say something, we use “tell”:
Paul told me that he doesn´t like lies ( Paul me contou que não gosta de mentiras)
I tell stories to my little sister every night ( Eu conto estórias para minha irmãzinha toda a noite)
“Talk” is often used to refer to a chatting between two or more people and in the most of the cases is followed for “to”:
The teacher talked to the students the whole class (O professor conversou com os alunos a aula inteira)
I talk to my grandparents every weekend ( Eu converso com meus avós todo final de semana)
“Speak” in other way is usually used to talk about the ability of speak another language or to talk to a group of people about an specific subject.
I speak English very well. (Eu falo inglês muito bem)
The teacher spoke to the class how it is important to study. (O professor disse a classe como é importante estudar)