Both will and going to express future meaning, but they're used in different situations. The key is your intention: spontaneous decision or prior plan?
will vs going to
Use
will
going to
Decision
Spontaneous (right now)
Prior plan or intention
Prediction
No evidence ("I think it will rain")
Evidence visible ("Look — it's going to rain!")
Promises/offers
"I will help you."
—
will: "The phone is ringing." "I 'll get it." (spontaneous)
going to: "I'm going to visit my parents this weekend." (planned ahead)
will: "It will be cold tomorrow." (general prediction)
going to: "Look at those clouds — it's going to rain." (evidence)
Common Mistakes
✗
"I will going to travel." → I will travel. OR I am going to travel.
✗
"Look — it will rain!" → it's going to rain! (evidence = going to)
Key Rules
will = spontaneous decision, promise/offer, prediction without evidence
going to = prior plan/intention, prediction with visible evidence
Present Continuous = fixed arrangements ("I'm meeting him at 3 pm.")