TIM is the easiest, expect about 10 minutes to fill the paperwork at the store and it can take up to 12 hours afterward to be activated.

10 euros for the activation, including 5 euros of prepaid credit. Afterward you can fill it in for up to 150 euros at a time, plus 5 euros for the refill charge.

And they will need to make a photocopy of your passport, don't forget to bring it at the store...