a + singular noun beginning with a consonant: a boy; a car; a bike; a zoo; a dog
an + singular noun beginning with a vowel: an elephant; an egg; an apple; an idiot; an orphan
a + singular noun beginning with a consonant sound: a user (sounds like 'yoo-zer,' i.e. begins with a consonant 'y' sound, so 'a' is used); a university; a unicycle
an + nouns starting with silent "h": an hour
a + nouns starting with a pronounced "h": a horse
In some cases where "h" is pronounced, such as "historical," you can use an. However, a is more commonly used and preferred.
In other words, if you say "You ka lay lee" then it is a ukulele.
If you say "oo koo leh leh" then an ukulele.
You guys don't look much like Jon to me.
Humph.