to tell you something
I want to tell you something.
Por cierto, tengo que decirte una cosa.
By the way, I have to tell you something.
I need to tell you something.
¡Rosalía, Rosalía, debo decirte una cosa ya!
Rosalía, Rosalía, I have to tell you something right now!
— Divorzio all'italiana (1961)
I came to tell you something.
Quiero decirte una cosa curiosa.
I want to tell you something curious.