Use "tell" for conveying information. Usually, you need to indicate who the listener is when you use "tell", eg. "tell me ___"

It's a little bit confusing but here is a way to differentiate between the two.. speak refers more for knowledge and use of languages.. for example I speak Arabic. can't say I tell Arabic.... Speak out. Speak up. while tell is used to inform or instruct... to tell someone to do something ..for example: Tell me the truth. Tell him more about it. Tell a story.

To tell is to give information to someone. To speak is to talk (you don't have to be giving specific information to another person).