We're going to play basketball this afternoon. - future plan
Look at those clouds! It's going to rain! - prediction based on present evidence.
In this case, we're going to practise with the second one, predictions.
Let's have a look at Rafael Nadal during practise at the Artois championship 2007 in London:
1. Watch the first part
Rafael Nadal - 1st part
2. What's going to happen next?
Rafael Nadal - 2nd part
Now, look at the photos and say what is going to happen.