Tell means to pass on information, e.g. secrets, or gossip.
If you tell someone to do something, you give instructions from a position of authority, and often use imperative language. Contrast ask, where a question is posed, possibly not from authoritative position.
This has perspective shifting, e.g. [me to my son] "Clean your room!", [me to my wife] "I've told him to clean his room".
Note that although "tell" usually catenates with a to infinitve, it's also possible to quote someone using an imperative form, e.g. "I told him to clean his room" (to-infinitive) v. "I told him - 'clean your room!'" (imperative). Note that the imperative form doesn't have perspective shifting.
Tell can also introduce a relative clause, e.g. "I told him (that) he should clean his room".
Idiom; "a little bird told me..."