Don't use un, une when stating people's professions (zero article)

Look at these equivalent sentences:

Il est avocat.He is a lawyer.

Marie est avocate.Marie is a lawyer.

Je suis vendeur dans un grand magasin.I am a sales assistant in a department store.

Paul est médecin.Paul is a doctor.

In French we don't say un or une (a) when stating someone's profession. 
We literally say "He is a lawyer", or "She is a doctor." 

Also note: the job description changes if the person is feminine or masculine (see gender of professions)

 
ATTENTION:

In French, you always need an article in front of a group including an adjective, and that rule applies to professions:

Lisa est une très bonne vendeuse.Lisa is a very good sales assistant.

Maurice est un professeur excellent.Maurice is an excellent teacher.

Vous êtes un mauvais avocat.You are a bad lawyer.
 

See also Job titles differ depending on whether you're a man or a woman (nouns)

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Examples and resources

Marie est avocate.Marie is a lawyer.
Je suis président. I'm a president.
Il est avocat.He is a lawyer.
Vous êtes un mauvais avocat.You are a bad lawyer.
Paul est médecin.Paul is a doctor.
Maurice est un professeur excellent.Maurice is an excellent teacher.
Je suis vendeur dans un grand magasin.I am a sales assistant in a department store.
Lisa est une très bonne vendeuse.Lisa is a very good sales assistant.
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