When we are talking about the future we are thinking about a particular condition or situation in the future, and the result of this condition.
There is a real possibility that this condition will happen. For example, it is morning. You are at home. You plan to play football this afternoon. But there are some clouds in the sky. Imagine that it rains. What will you do?

IF condition result
present simple WILL + base verb

If it rains I will stay at home.

Notice that we are thinking about a future condition. It is not raining yet. But the sky is cloudy and you think that it could rain. We use the present simple tense to talk about the possible future condition. We use WILL + base verb to talk about the possible future result.

The important thing about the first conditional is that there is a real possibility that the condition will happen. Remember the two basic structures:

IF condition result and result IF condition

IF condition result
present simple WILL + base verb

If I see Peter I will tell him.
If Susan is free tomorrow we will invite her.
If it rains tomorrow what will you do?

result IF condition

WILL + base verb present simple

I will tell Peter if I see him.
He will invite Susan if she is free tomorrow.
What will you do if it rains tomorrow?

Sometimes, we use shall, can, or may instead of will, for example: If you are good today, you can watch TV tonight.