Today I'm making plans for my next weekend, If it is sunny I will go to the beach. But I need to buy a lot of things, specially sunscreen, because if I don't use it I get sunburned after swimming some hours.
Conditional Clauses have two sentences:
The conditional clause that refers to a condition to get a result.
The main sentence that refers to the result.
This conditional is used when the result will always happen.
It is used to talk about habits, scientific facts, general truths, instructions and rules, if something else happens first.
In zero conditional sentences, you can replace "if" with "when", because both express general truths.
The first conditional is used to express the future consequence of a realistic possibility now or in the future.
For example, If I miss the train, I'll take the next one. There is a 50% chance that the first part of this sentence (the action following 'if') will happen.