We use indefinite articles when we refer to general or non-specific things (not mentioned before) unknown to both the speaker and the interlocutor, and only with singular countable nouns.
Also, we use them when we talk about jobs and professions.
- I am a doctor.
I am an engineer.
- She has got a pear.
She has got an apple.
- We saw a huge elephant in the park .
We saw an elephant in the park .