One of the biggest deals to learn as a beginner is the difference between SER and ESTAR.
Let's forget the differences between the two and just look at ser, and when/how to use it.
SER: to be
SER is really just to talk about the hard facts. Nothing at all temporal or changing. But don't take that too literally. You can say for example, É um bom dia para a praia. ➜ It's a good day for the beach; when we all know that the weather could change in 10 minutes from now.
You will hear Brazilians using estar in a sentence like this just as often. Just try to feel the substance of ser in these examples:
We all know that nationality is pretty solid. You have to use ser.
If you wanted to say that someone is being well organized in the moment, even though in general they are not you would use estar and change the sentence to something like "Você está sendo bem organizado(a) agora".
Personal traits, characteristics as always expressed with ser -- unless they are constantly changing. If a person is fundamentally a happy person for example, you would use ser to say "Ela é muito feliz". And then if that changed for some moment you would use estar to say "Ela está triste."