Except for facts we cannot change (I'll be eighteen tomorrow.), WILL is usually not as definite as BE GOING TO, and is often used when we promise, offer to do something (often spontaneously), or when we are making a decision.  
It's a long way to the city center. It'll take you hours. Get in the car. I'll give you a lift.

GOING TO is more definite. It expresses an intention, when we have already made a decision, or when we know something is going to happen.  
I'm going to study law. I want to be a lawyer.

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