We don't use the indefinite article un, una when talking about professions.
Read and listen to these examples:
Yo soy profesor.
I am a teacher.
Tú eres peluquera.
You are a hairdresser.
Pablo es panadero.
Pablo is a baker.
Marina es jardinera.
Marina is a gardener.
Notice that we literally say:
Yo soy carnicera.
I am a butcher.
Also note that most of the time the job titles are different for men and women. Here is a list of ordinary professions.
However, if you are giving extra information adding a quality adjective, you always need article "un/una" in front:
Yo soy un mecánico bueno.
I am a good mechanic.
Elisa es una cocinera mala.
Elisa is a bad cook.
See also Artículo cero o ausencia de artículo and Artículo
Artículo cero o ausencia de artículo
Pablo es panadero.
Pablo is a baker.
Yo soy profesor.
I am a teacher.
Elisa es una cocinera mala.
Elisa is a bad cook.
Yo soy un mecánico bueno.
I am a good mechanic.
Marina es jardinera.
Marina is a gardener.
Yo soy carnicera.
I am a butcher.
Tú eres peluquera.
You are a hairdresser.
