Future actions
Study the difference between will and (be) going to:

Sarah is talking to Helen:
SARAH: Let's have a party
HELEN: That's a great idea. We'll invite lots of people.
will ('ll): We use will to announce a new decision. The party is a new idea.


Later that day, Helen meets
Dan:HELEN: Sarah and I have decided to have a party. We
're going to invite lots of people.
(be) going to: We use (be) going to when we have already decided to do something. Helen had already decided to invite lots of people before she spoke to Dan.

Compare:
- 'Gary phoned while you were out.' 'OK. I'll call him back.'
'Gary phoned while you were out.' 'Yes, I know. I'm going to call him back.' - 'Anna is in hospital.' 'Oh really? I didn't know. I'll go and visit her.'
'Anna is in hospital.' 'Yes, I know. I'm going to visit her this evening.'
Future happenings and situations (predicting the future)
We use both
will and
going to to predict future happenings and situations. So you can say:
- I think the weather will be nice later. or
I think the weather is going to be nice later. - Those shoes are well-made. They'll last a long time. or
Those shoes are well-made. They're going to last a long time.
When we say something
is going to happen, we know this from the situation
now. What is happening
now shows that something
is going to happen in the future. For example:
- Look at those black clouds. it's going to rain. (not It will rain)
(We can see that it is going to rain from the clouds that are in the sky now.) - I feel terrible. I think I'm going to be sick. (not I think I'll be sick)
(I think I'm going to be sick because I feel terrible now.)
Do not use will in this type of situation.