Usually, nouns that end in -o are masculine and nouns that end in -a are feminine. For example:
There are some exceptions to this rule, eg planeta, día, mapa and tranvía are all masculine, whereas mano and foto are feminine.
It can be hard to tell if some nouns with other endings are masculine or feminine. For example, nouns that end in -e could be either masculine, eg hombre or feminine, eg leche.
Some words also mean entirely different things depending on their gender, for example:
Are these nouns masculine or feminine?
Make these nouns plural.