Many Roman bars from 18:00–21:00 offer drinks (10–15€) with free buffet snacks. A budget pre-dinner option.
Trastevere and Monti have the best aperitivo culture. Freni e Frizioni and Salotto 42 are classic Trastevere spots.
Drink a Negroni or Aperol Spritz to fit in. White wine is also acceptable; cocktails with vodka or rum mark you as Northern European.