Depends. The organiser of the event may have listed Free Tickets to their event, in which case you’ll need to redeem a ticket in the same way as if it were a paid ticket (see: How do I buy a ticket on Billetto?).
If the organiser has created a Guest List, you will only need to sign up to add your name to the list (which is the same process as described for the answer to How do I buy a ticket on Billetto? but you will not receive a ticket by email).