In Spanish nouns have implicit the gender of the referred object. A lot of Spanish nouns end in a to show that are feminine, and the ones that end in o are masculine, as a general rule. There are exceptions like the noun águila (eagle), which is feminine, but it´s treated as masculine to avoid two a together (the one from la and the beginning sound of águila ): el águila.
You have to use the appropriate article and pronoun :