¡Yo no fui!
You probably learned that that this translates to "I didn't go". And that would be 100% correct.
¿Fuiste el super?
Did you go the supermakert?
No, I didn't go.
But as always, phrases in Spanish typically have more than one meaning.
If someone is trying to blame you for something, the second definition of "yo no fui" is something you'll want to remember:
Quien dejó la puerta abierta? - Who left the door open?
Quien quemó el pollo? - Who Burned the chicken?
Quien hizo eso? - Who did that?
¡¡Yo no fui!!
It wasn't me
In fact, I recommend you blame you someone else...
Yo no fui, ¡ fue Anthony!
It wasn't me, it was Anthony
Of course if it’s something good like…
Quien limpio la cocina? - who cleaned the kitchen?
I would say “Fui yo!” - It was me!