Great question!
Tell = Give information. Sometimes it even means receive information (eg "I can tell what you mean" = "I see/know what you mean"). Sometimes it's not even about information (eg, in a bank, the teller is the cashier, the person giving money to customers).
Talk = Say things
I am talking = I am saying things = I am speaking
But there is a slight difference of emphasis between these words.
"Speak" emphasises the mechanics of opening your mouth to talk. So if you're inappropriately silent, I may say "Speak, please!" or "Open your mouth and say something!"
"Say" is half way between Speak and Tell. It means provide the information (like Tell) but generally by mechanically speaking (like Speak).
These words also often need different grammatical structures in their sentences.
I apologise for this answer being so poorly composed. The differences are so nuanced that they're hard to explain accurately. Generally, native speakers don't think much about it, we just naturally become familiar with how these words are typically used.