Letters are either vowels or consonants. In grade school, you learned that vowels are a, e, i, o, and u, and sometimes y. The word consonant refers to the sound produced by occluding with or without releasing (p, b; t, d; k, g), diverting (m, n, ng), or obstructing (f, v; s, z, etc.) the flow of air from the lungs. When spelling, sometimes you'll double a consonant ending a word before adding a suffix, as I just did when I doubled the consonant “l” in spelling.
When you use the indefinite article a and an, you'll use an when the word modified begins with a vowel sound. Pay no attention to the letter beginning the word. The sound is what's important. Thus:
a university an umbrella a historic event an hour