First of all, let's be VERY clear: THERE ARE NO RULES WITHOUT EXCEPTIONS WHEN IT COMES TO PLURALS IN ENGLISH. Some plural forms of nouns you just have to learn by heart (memorize).
HOWEVER: here are some basic rules to help you learn how to form plurals for most nouns.
1.) Most of time, just add s. flowers
2.) Add es if the word ends in o. potatoes
3.) Add es if the word ends in ch, sh, x, z, or s. glasses, churches, boxes, flushes
4.) Add s if the word ends in y with a vowel before it. monkeys, stays
5.) Add ies if the word ends in y with a consonant before it. parties
Master these five before you try the next three. These rules are not used very often.
6.) Add s if a proper noun (a name) ends in y. the McKinley family -- the McKinleys
7.) Add s to the main noun in a compound noun, which has more than one word. brothers-in-law
8.) Add s to the end of the end of compound noun with no main noun. follow-ups, trade-ins
Source: Painless Grammar by Rebecca Elliott, Ph.D., 1997