Use a or an before a non-specific singular countable noun. Choose according to the next sound: a before a consonant sound and an before a vowel sound.
Practical examples
I need a pen.
أحتاج إلى قلم.
She is an engineer.
هي مهندسة.
We have an hour before the meeting.
لدينا ساعة قبل الاجتماع.