The plural is formed in Catalan by adding an s.
However in many cases, the stem of the noun also changed. The following are some basic rules illustrating the more common changes.
If the singualar ends in an unstressed a then the plural normally ends with es.
If the singualar ends in a stressed vowel then add ns to form the plural.
A large number of masculine nouns ending in s, ç or x add os to the stem.
Catalan grammar is relatively straight-forward for English speakers. It doesn't have cases and inflection like the slavic languages (E.g. Polish or Russian), but there are still numerous exceptions.
The rules for forming Catalan plural nouns demonstrate this perfectly, for example with masculine nouns ending in s, ç or x.
These rules give an idea of how plurals are formed, but the only way to actually do this accurately is through continued exposure to the language whether that be through reading, listening or speaking.
For example, you are unlikely to need to learn that the plural of problema is problemes because you will encounter the word frequently. The above rules help identify patterns which you can later use.