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.