How do you know the gender of French words?

In English we only use genders for people. In French everything has a gender and knowing it will allow you to use the right article, pronouns and ending of adjectives.
There are no absolute rules on how to determine gender. However you will find that words with the same ending usually have the same gender. It is the case for all nouns finishing in -eur which are typically masculine words, while -euse is feminine.