Hi Stella,

"It's so confusing"    Wow!   --  I've lost count of how many times I've used that very expression in the many years that I have been studying the language.

Un and une are indefinite articles and there is a third -- des.

They replace a, an, some/any in English -- un with masculine singular nouns, une with feminine singular nouns and des with plural nouns.

Hope this helps.

Bonne continuation.

Jim

Just to complement Jim's answer:

J'ai un stylo. -- I have a pen. But also: I have one pen.
J'ai le stylo. -- I have the pen.
J'ai des stylos. -- I have some pens.