Sou (verb "ser"): used for permanent situations like nacionality, professions, personal attributes etc. Examples: Eu sou brasileiro (I'm brazilian); eu sou advogado (i'm a lawyer); eu sou uma pessoa divertida (I'm a funny person).
Estou (verb "estar"): used for transitory situations like where you are, diseases, emotions etc. Eu estou em casa (I'm home); Eu estou com febre (I've got a fever); Eu estou triste (I'm sad).
I think your doubt is caused by the fact that in english, you use only the verb to be for the both situations (permanente and transitory). But in portuguese we have two different verbs...