Step 1

Articles are more common in Spanish than in English. In English some examples of articles are: “the” (definite), “a”, “an” and "some" (indefinite).

There are 4 indefinite articles ( “a/an” & “some”). Indefinite articles refer to an unknown person or thing.

un – a / an (masculine, singular)
una – a / an (feminine, singular)
unos – some (masculine, plural)
unas – some (feminine, plural)

Examples:
Es un gran museo.
     It's a big museum.
Soy una profesional.
     I am a professional. (female)
Estoy con unos amigos.
     I'm with some friends.
Son unas grandes estancias.
     They are some big farms.

Note!
uno is not an indefinite article; it is a pronoun. It never accompanies a noun.