When we want to say what we do for a living we use the verb ser followed by a profession. For example:
Notice that the adjective that describes the profession needs to agree in gender and number with the subject.
Also notice that in Spanish we do not add the indefinite article un/una [a] before the profession. Have a look:
¿Tú eres una azafata? -Sí, soy una azafata.
Are you a stewardess?-Yes, I am a stewardess.
Are you a stewardess?-Yes, I am a stewardess.
To see how verb estar can sometimes be used to talk about jobs that we don't consider as defining us, or as our profession, see Using estar (not ser) + de for a temporary job.
See also Conjugate ser in El Presente (present tense).
