Use 'say' to to speak in general about something that has been said by someone. 'Say' is often used to report what someone else has said.
John said he had a good time in Las Vegas.
The teacher often says we need to study more.
Important Note: 'Say' refers to any type of speech and is therefore more general in nature.
Verb Forms: Say - Said - Said - Saying
Use 'tell' to mean that someone has instructed or informed someone else of something. 'Tell' is often used to report what someone else has told a specific person.
Angela told them to hurry up.
Our friends told us about their experiences in Germany.
Important Note: 'Tell' is always followed by an indirect object. The infinitive form is often used following the construction to indicate instructions (see example above).
Verb Forms: Tell - Told - Told - Telling