Most English speakers will use the correct indefinite article (or determiner, if you prefer) without even thinking about it.
If you find yourself worrying, this is how to know which is correct:
A precedes words that begin with consonant sounds (letters that are not a, e, i, o or u).
An precedes words that begin with the vowel sounds a, e, i and o.
The letter u is different depending on the sound. If the beginning of the word sounds like ‘you’ (or ‘yoo’), use a. If the word begins with ‘uh’, use an.
The letter h may also vary depending on sound. If the word starts with a hard h sound, use a. If the word starts with a silent letter h, use an.
It is the sound that determines which indefinite article you should choose: a eucalyptus tree; a one-off; an understandable choice; an honourable man. Let the sound guide you when applying to single letters or groups of letters: an FAQ; an SAS unit; an MA; a B road; a TUC member; a UFO.