"Amigo" is used for a male friend, while "amiga" is used for a female friend.
You can also use the plural forms "amigos" (male friends) and "amigas" (female friends).
For example:
* "Meu amigo João" - My friend John
* "Minha amiga Maria" - My friend Mary
* "Seus amigos" - Your friends (male)
* "Suas amigas" - Your friends (female)