Use 'a' with nouns starting with a consonant (letters that are not vowels),
'an' with nouns starting with a vowel (a,e,i,o,u)
An before an h mute - an hour, an honour.
A before u and eu when they sound like 'you': a european, a university, a unit
The indefinite article is used:
Notice also that we usually say a hundred, a thousand, a million.
NOTE: that we use 'one' to add emphasis or to contrast with other numbers:
I don't know one person who likes eating elephant meat.
We've got six computers but only one printer.