When a word ends in "y" preceded by a consonant, you should usually change the "y" to "i" before adding the suffix:

curly becomes curlier

party becomes parties

thirty becomes thirties, thirtieth

However, if the suffix already begins with "i", keep the "y" (except before the suffix "-ize"):

thirty becomes thirtyish

fry becomes frying

agony becomes agonize

memory becomes memorize 

When the ending "y" is preceded by a vowel ("a" "e" "i" "o" or "u"), "y" does not change to "i":

journey becomes journeying

trolley becomes trolleys