Wah How to use first conditional.

We use first conditional when we talk about actions in the future that are not happening now but are likely to happen in the normal turn of events.

We use If + present simple tense to talk about the possible future condition.
We use will + verb to talk about the possible future result. 
    If it rains tomorrow, I will stay at home.
    If he spends all his money, he won't be able to buy a new car.
    If you feel better tomorrow, Will you go to the sea?

First conditional sentences are normally reversible.
    I will stay at home if it rains tomorrow.
    He won't be able to buy a new car if he spends all his money.
    Will you go to the sea if you feel better tomorrow?

We can use going to for the events that are going to happen.
    If you don't hold that handrail, you are going to fall off.
    You are going to get wet if you run through the rain.
    If she doesn't stop eating junk food and sitting all day, she is going to be fat for her favorite dresses.

Note: We can use can, may instead of using will.
    If you study hard right now, you can easily pass the examination.
    I may help you if you beg me. 

We can play rainwater if it rains this afternoon.

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"

Wah How to use "Won't for Refusal"

We know how to use Will and Won't but we can also use won't to refuse someone to do something.

    I won't give you any advice. (I refuse to give you any advice.)
    He won't listen to you. (He refuse to listen to you.)
    I won't eat this hamburger, it's not good enough for a dog!
    She won't give me her name.
    They won't come the party tonight,it's a pity.

"It's not over tonight
Just give me one more chance to make it right
I may not make it through the night
I won't go home without you..."  - Maroon 5 - Won't Go Home Without You

Wah How to use "Will"

We can use "will" to talk about the future. We also use will to predict, to promise, to request, make a threat and talk about decision.

Affirmative           
    I will come back later. (I'll come back later.)
    He will come to the dinner. (He'll come to the dinner.)

Negative
    I will not come to the dinner. (I won't come to the dinner.) 
    I will not go away from you. (I won't go away from you.)

Interrogative
    Will you come to the party tonight?
    Will it rain this afternoon?         

1. We use will to talk about the future.
    I'll be back next two weeks.
    I'm in the meeting right now, I'll call you later.

2. We use will to make predictions.
    I think it'll snow tonight.
    I'm sure he'll be a successful manager one day.

Note: I don't think it'll snow tonight  I think it won't snow tonight. is incorrect.