Questions often arise regarding words that start with h and u because words that start with those letters often have both consonant and vowel sounds. Think hour (I'll be there in an hour), historic (It was a historic moment), unicorn (I've never seen a unicorn), unfair (She had an unfair advantage), and so on.
The letters o and m can be tricky as well. Just like with the letter u, you would think that you would use an before a word that starts with o. This is not always the case. You would say "He's not just a one-trick pony," because one-trick starts with the w sound. Similarly, "She has an MBA, but chose to work as a musician."
See what I did there? I threw an abbreviation in there. These can be most confusing for people. When you're dealing with an abbreviation, your choice of a or an depends on whether you pronounce the expression letter by letter (initialisms) or as a word (acronyms). Initialisms that start with the letters F, H, L, M, N, R, and S are preceded by an. When these same letters introduce an acronym and are pronounced as part of a word, they are preceded by a. Examples: an F&B expense / a FAM trip, an RSVP / a RevPAR report, and so on.
Complicated? A bit, yes. All you need to remember is that it is the sound that determines whether you use a or an, not the actual first letter of the word. I hope this helps!