Many singular nouns are pluralised by simply adding -s, but there are a number of exceptions.
Nouns ending in -ch, -sh, -x and -ss usually add -es.
church / churches
box / boxes
Nouns ending in -y usually change to -ies, unless there is a vowel before the -y.
fairy / fairies
donkey / donkeys
Nouns ending in -f or -fe usually change to -ves.
shelf / shelves
wife / wives
Some nouns change in different ways or don't change at all!
child / children
foot / feet
sheep / sheep