They are a lot more complicated than in languages like English, Spanish, Italian etc.
Basically, sometimes you add an ending e.g. afal > afalau (apples), gair > geiriau (words).
Sometimes you take an ending away e.g. llygoden > llygod (mice), deilen > dail (leaves).
Sometimes you take one ending away and add another e.g. cwningen > cwningod (rabbits), cerdyn > cardiau (cards).
As you can see, some of the can include vowel changes too. In fact, sometimes a vowel change is all you need e.g. car > ceir (cars), castell > cestyll (castles).
There are a few patterns, buts lots and lots of endings, so the advice given is to learn the plural for each word as you go along.
Watch out for irregulars too e.g. ci > cŵn (dogs).