Spanish has two verbs where English has one. Ser is about characteristics, and it is where es and son come from. Estar is about states, and the English word state is related to it.
So Spanish permits you, and obliges you, to make a decision every time: is this a state, or is it a characteristic?
Testing it out: being tired is a state. Where you are is a state. Being a good or bad person is a characteristic.
Most learners, and many teachers, reduce this to temporary against permanent. Snub out that temptation now. It works most of the time, but not all of the time.
The example that breaks it: I am a student is not very permanent, but your profession is, according to Spanish, characteristic of you. Even if you do it for five minutes.
Being aggressive is the same case: someone can be aggressive and then change, so it is not permanent, but it is still a characteristic.
Some adjectives can be used with both, giving different meanings, but the great bulk go with one or the other.
Two irregularities, and that is all. I am is estoy, ending -oy like voy.
For a verb meaning to be, this is remarkably tame. English gets am, is, are, was out of to be.
The second irregularity is worth its own note. Estar is the only verb in the entire Spanish language that does not put the accent on the penultimate syllable in the present tense.
estoy, not "éstoy"
The accent goes on the ending. This has not happened until now, and it will not happen again. The "we" form is the exception to the exception: estamos has the accent where you would expect, because you are adding two syllables.
Here the noun is the complete "to" form itself. Un ser is a being.
There is no rule that gets you to humano, but humanity gives humanidad, and from there you can guess humano.
Tired is a state, so it takes estar. That holds even when you say always, because the question is never how long it lasts.
Estoy cansado · Estamos cansados · Estamos cansadas · Estoy cansado siempre · Estoy siempre cansado · Siempre estoy cansado
Siempre is very flexible in that sentence. Moving it changes the emphasis but it is legal anywhere.
Death is a state too, so I am dead is estoy muerto. It also works as a way of saying I am very tired.