You can hail taxis just like you would in New York, or if you leave a restaurant sometimes they will call a taxi friend for you (we mostly walked but one time we asked for a cab). To get back to the airport our Airbnb host got a friend to drive us for $25 (what a taxi would cost). Price it before you get in. We always paid $5 regardless of where we were going in Havana (except the airport). No one asked us for anything more.