Rule 1: 'a' or 'an' for any one thing (not specific)
Use 'a' or 'an' when you talk about one thing for the first time, or when it does not matter WHICH one.
I want a cup of tea. (any cup)
She is a teacher. (one of many teachers)
He has a brother. (one brother)
Use 'an' instead of 'a' when the next word starts with a VOWEL sound (a, e, i, o, u).
An apple. An egg. An old chair. An hour. (h is silent, so it sounds like 'our')
A house. A car. A teacher. (consonant sounds at the start)