Textbooks teach "will" as the future tense. In real speech it is often the least used of the four.

will — decided right now

Use it for decisions made at the moment of speaking, offers, and predictions.

  • "The phone's ringing — I'll get it." (deciding now)
  • "I'll help you with that." (an offer)
  • "I think it'll rain." (a prediction)

going to — already decided

For plans made before the conversation, and for predictions with visible evidence.

  • "I'm going to start a course next month." (decided last week)
  • "Look at those clouds — it's going to rain." (evidence in front of you)

present continuous — fixed arrangements

For things already scheduled, usually with other people.

  • "I'm meeting the client at four."
  • "We're flying to Chennai on Sunday."

This is the most common future form in everyday speech, which surprises most learners.

present simple — timetables

For fixed schedules you do not control.

  • "The train leaves at 6:15."
  • "The office closes at seven."

Putting it together

"I'm meeting Ravi tomorrow" (arranged), "I'm going to ask him about the invoice" (my plan), "I think he'll say yes" (prediction), "his flight lands at nine" (timetable).

If it is already in your calendar, use the present continuous. That single habit makes your future English sound far more natural.