Add "es" to the end of the word. You can pluralize most nouns that end in an "s," such as dress, gas and lens, by adding an es to it.
Add an "i" to the sentence if it ends in "us." Most nouns that end in "us," such as fungus, stimulus and alumnus, require English speakers to replace the "us" with "i." There are a few exceptions, however, such as the word virus, which takes "es."
Note that proper nouns, such as the name of a place or person, do not take an "es" or "i" to pluralize them. Instead, leave them as they are. For example, write "the Jones" (not the Joneses) are coming to dinner.