BE GOING TO :

The structure be going to is mainly used:

• to talk about plans and intentions
• to make predictions based on present evidence

It is most commonly used in conversational English.

subject	be	going	full infinitive
I	am	going	to paint	the door.
You	are	going	to love	this book.
Jack	is	going	to buy	a new computer.
It	is	going	to rain	very soon.
We	are	going	to visit	a museum.
They	are	going	to accept	our invitation.

PLANS and INTENTIONS : (what we have decided to do)
Be going to is used to talk about plans and intentions, usually when the decision has already been made.

♦  I've made an appointment with my solicitor/lawyer. I'm going to ask him for some legal advice.
♦  Alex and Eva have announced that they're going to get married.
♦  Emma is going to share an apartment with Melanie.
♦  Pedro is going to revise his English all evening.
♦  I'm going to watch the match on television.

PREDICTIONS : (statements about the future based on evidence)
We use to be + going to + infinitive when we make a prediction based on evidence we have now, or when signs show that something is likely to happen.
For example, we can predict that the girl who is entering the church wearing a long white dress is going to get married.

♦  Diego is going to fail the exam if he continues to miss classes.
♦  Look at those clouds. It's going to rain!
♦  The ladder is shaking. That man is going to fall!
♦  She's playing much better than her opponent. She's going to win!
♦  Get back! Get out of here! The bomb is going to explode!
♦  It's late! Hurry up!  We're going to miss the bus.

GONNA :
Although it is grammatically incorrect, going to is sometimes shortened to gonna, in very informal conversations, especially when we speak quickly. Gonna is also used in song lyrics.
Gonna is more often found in American English, but it is never written that way (except in song lyrics).

♦ I'm gonna surprise you all one day!  =  I'm going to surprise you all one day.