"Tell" is nearly always transitive and can have one object.
- He told a lie. He told you a different story than he told me.
"Tell" can be used to give a command.
- Tell me what you know about the merger.
- She said, "I can't help you." She said that she couldn't help me.
"Speak" and "talk" are similar but "talk" refers more to a conversation rather than one person's statements.
- We talked about our holidays for hours.
- The doctor spoke to us about the test results.
- Some animals can talk or say things but they are not capable of telling us anything!