BE GOING TO :
The structure be going to is mainly used to talk about plans and intentions, or to make predictions based on present evidence. It is most commonly used in conversational English.

In very informal spoken English, going to is sometimes shortened to gonna.

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 :

Be going to is used to talk about plans and intentions, usually when the decision has already been made.

♦ 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 :

We use be going to when we can see that something is likely to happen.

♦ Look at those clouds. It's going to rain!
♦ The ladder is shaking. That man is going to fall!
♦ 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.