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.