A cashier is correct because the article "an" is used before words starting with vowels(a,e,i,o,u) or with vowel sounds.The article "a" is used before consonants.
E.g- An elephant ( word starting with vowel )
An honest boy ( work starting with vowel sound - honest )
And so in this case,(cashier) we will say - a cashier.
Hope it helped :)
Mark it best ^_^