Wah How to use "Be going to"

To talk about the future we know how to use will in this topic
Wah How to use "Will"

We use be going to for decisions or plans we have made about the future.

    I am going to go to Washington, D.C. next week.
    We are going to climb that mountain.
    Are you going to attend the meeting tonight?
    What are you going to do for your vacation?

If there is evidence for the future event. 
You can see the future because of what happening now.
You can use be going to. 
    There are a lot of clouds, it's going to rain soon.
    The traffic is very dreadful. I am going to be late.
    Look at the queue. We're not going to get in for over two hours.

Going to compare to Will 
    There is someone knocking at the door, I will open it and see. (offer)
    I am going to work out at a gym this evening. (plan)

Note : I'm going to have lunch here you can also shorten it to I'm gonna have lunch here.

You can use be going to in past tense in this topic
Wah How to use "Was / Were + going to"