When something is not yet known, use the indefinite article a or an. When the next word begins with a consonant sound (e.g. a mother), the indefinite article is a. When the next word begins with a vowel sound (e.g. an aunt), the indefinite article is an.

Is there a code for that door?

Remember to use the definite article the when talking about something that is already known.

The man opens the door.

Uncountable nouns such as water don’t normally get an indefinite article.

He wants water.