Anything with a vowel at the start (a, e, i, o, u) EXCEPT when the sound is not that of a vowel. Since words like "uniform" and "university" are pronounced with a "y" to start (yoo-ni-ver-sity, yoo-ni-form) you say 'a university' not 'an university'. Older guides will also tell you do put "an" before words starting with an H - "an hospital", "an historic day" but this is old-fashioned.

'An’ should be followed by a vowel (a,e,i,o,u), and 'a' should be followed by a consonant. But this is not always true! When using either of them, pay attention to pronunciation, and not on how the noun is written: -The word hour has a soft ‘h’ which is weakly pronounced, and therefore we say ‘an hour’ -The word house has a hard ‘h’, so we pronounce 'a house' Said otherwise, use 'an' before nouns that start with a vowel when pronouncing them, and 'a' before everything else.

An is used before a noun starting with a vowel ie: an apple, an egg, an igloo, an orange, an umbrella. Any noun basically starting with a, e, i, o or u, simple huh! :)

The rule is "an" comes before a vowel SOUND. It's a spoken rule rather than a written rule.