Going to can express intentions, things we have previously decided to do (I am going to watch TV) or predictions, if we have a clue that something is going to happen: the sky is dark and cloudy: It is going to rain).
Will deals with instant decisions (things we decide to do on the spur of the moment): I�ll close the window, when you see that someone in the room is cold. It also expresses predictions, but only when we are just guessing and have no clue whatsoever about what is coming up: Barcelona will win the World Cup.
So, you could actually use think with both:
I think it�s going to rain (if you are looking at the cloudy sky)
or
I think I�ll stay at home this evening (if you don�t want to get wet!)
I hope this helps.
Great site, Ana. Thank you!!
