In most cases, to make a singular noun to a plural noun, we add an 's' or 'es' after the word.
However, as you can tell from the word 'jeans' there are some words that never have a singular form. Which means that even if there is just one item, we always use the plural form (jeans). If the word jeans is always plural, it also means that the noun takes on a plural verb ('are' not 'is').
As I've mentioned many times before, English can definitely get confusing and it is near impossible to memorize everything. The best way to learn is to listen and speak more! It will start to become more natural for you!