Talk and speak can be used interchangeably. Speak is often used more for important scenarios. i.e. "I have to speak with the school counselor." or "I must speak with the community this afternoon"
You usually use talk in every day situations. "Hey, want to talk to my friend, Paul?" or "Can we talk later this afternoon?"
You can say "Can we speak about something later this afternoon" as well, but using talk sounds more natural.
Tell has a couple different meanings. Say you're hiding something from someone and decide to >tell< them about your secret. "I have to tell you something" would be the best way to start the conversation. Another case is to "tell the difference" between a couple of things. For example, "Can you tell the difference between talk to and speak to now?"