Both of these words have the same translation into Spanish, but in English, they’re used differently.

Only is the more common of the two.  It means there’s just one of something

Unique has the same definition, but in this case, it’s not just the only copy, it’s special in some way.

For example:

I have only one car.
She is the only blonde girl in the class
I only like red M&Ms.

She has the unique ability to learn languages extremely quickly.
This diamond has a unique color.
The 1959 Cadillac is unique; no car has ever been so big or so elegant.

So, “only” when there’s just one of something but it’s not that special, and “unique” when there’s only one of something, and it’s very special!