When we want to say that there are more than one of something, we use plural.
Plural of nouns ( = множина именица) can be:
regular - add - s or - es to a singular noun
irregular - learn a list by heart
By the way, some nouns have the same singular and plural forms (fish, sheep, deer, species), others have only the singular (information, advice, news) or the plural form (trousers, scissors, means). In compounds, the plural ending is added to the main noun (passers-by, theatre-goers).
For more visual explanation, watch this video: