Jobs in German can be masculine or feminine, like the people doing them. When a job is done by a woman, very often an -in ending will be added.
Ich bin Lehrer I'm a teacher (male)
Ich bin Lehrerin I'm a teacher (female)
Ich bin Polizist I'm a police officer (male)
Ich bin Polizistin I'm a police officer (female)
Ich bin Mechaniker I'm a mechanic (male)
Ich bin Mechanikerin I'm a mechanic (female)
When you say what you do in German, you don't need to use the word a.