We add -s to words for two reasons:

to make plural nouns (boy > boys)
to form the 3rd person singular of the present simple tense (I work > you work > he works)
Spelling Rule
Just add -s to the end of the word, for example:

dog > dogs
play > plays
demand > demands
Exceptions

If the word ends in:
do this:
and add:
For example:

-ch
-s
-sh
-x
-z
nothing
-es
church > churches
mass > masses
brush > brushes
fax > faxes
box > boxes
chintz > chintzes

-f
-fe
remove the -f or -fe
-ves
wife > wives
calf > calves

except:
beliefs, chiefs, dwarfs, griefs, gulfs, proofs, roofs

consonant + -y
remove the -y
-ies
spy > spies
baby > babies

Note: words that end in -o normally just add s, except:
buffalo > buffaloes
cargoes (or cargos)
domino > dominoes
echo > echoes
go > goes
grotto > grottoes
halo > haloes
hero > heroes
mango > mangoes
mosquito > mosquitoes
motto > mottoes (or mottos)
potato > potatoes
tomato > tomatoes
tornado > tornadoes
torpedo > torpedoes
veto > vetoes
volcano > volcanoes