The basic rule for pluralising a noun is to add s, but there are many exceptions to this rule. Some nouns do not change at all (such as sheep), while others change completely (such as person / people).
Separate rules often apply to words that we have borrowed from other languages. All the following nouns come from Latin. The first group adds e instead of s to make the plural.
The plurals of these nouns are formed by deleting the us and adding i.
This group forms the plural by deleting the ix or ex and adding ices.
Sometimes you can work out the plural of a new word by comparing it with other words that you already know, but be careful, as there are always exceptions!