The Spanish word Un(o)/Una has two possible translations into English, the number “one” and the article “a/an”.  They’re often confused, because oftentimes, people don’t know which is appropriate for the context.

“One” is a number or a pronoun.  We use it when we want to specify that there is only one of something, or to refer to an indefinite person, either by itself or as part of a larger pronoun.

Number:
Missy has two cars, but I only have one.
I only have one copy of “The Lord of the Rings” on DVD.

Pronoun:
Jenny isn’t one to tell lies.
Give it to anyone who wants it.

“A” or “an” are indefinite articles. Before singular countable nouns, these words are much more common than “one”.  We use them when the fact that there is only one of something doesn’t matter.  These articles also serve as a marker that this is the first time this particular noun has come up in conversation.

I have a car.
I do have a copy of “The Lord of the Rings” on DVD—would you like to borrow it?

And:
I saw a man at the fair.  The man was wearing a suit!

In the last example, we can see that the first time the man is mentioned, we use “a”, but the second time, we use “the”, because by then, he’s not new anymore.  But the “a” before man signals that this is the first time I’ve mentioned him; likewise, we can assume that since I didn’t use “a” with “the fair”, the speaker and the listener know what fair is being referred to.