to be: SER (as in "I am a boy - eu sou um rapaz) this verb indicates something permanent Present simple conjugation:

Eu Sou
Tu És
Ele/Ela É
Nós Somos
Vós Sois
Eles/Elas São

to be: ESTAR (as in "I am at the cinema" - eu estou no cinema) this verb indicates something temporary Present simple conjugation:

Eu estou
Tu estás
Ele/Ela está
Nós estamos
Vós estais
Eles/Elas estão

Note that vós is no longer in use by any Portuguese speakers. Tu is used only in Portugal between very close family members, or friends. Portuguese still uses the future subjunctive tense, unique amongst other Romance languages.