If you have dietary restrictions, it's polite and necessary to communicate them clearly. To let staff know you don't eat meat, say Sono vegetariano (I am vegetarian, masculine) or Sono vegetariana (feminine — change the final "o" to "a"). For a gluten allergy, use Sono allergico al glutine (masculine — again, swap to "allergica" if you're a woman). To request a milder dish, say Senza piccante, per favore (without spicy, please).
When it's time to settle the bill, the phrase to remember is Il conto, per favore (the check, please). Before paying, it's worth asking Quanto costa? (how much does it cost?) if a price isn't listed. To check payment methods, ask Accettate carte di credito? (do you take credit cards?) or Posso pagare con la carta? (can I pay by card?). Most Italian restaurants accept cards, especially in cities and tourist areas.
If something goes wrong with your meal, it's helpful to know a few feedback phrases. È troppo salato (it's too salty), È troppo cotto (it's overcooked), and È poco cotto (it's undercooked) cover the most common complaints. Politely point out the issue, and most kitchens will gladly fix it. Don't forget the basic needs, like Dov'è il bagno? (where is the bathroom?) — Italian restaurants generally have clearly marked bathrooms, often down a flight of stairs.
Finally, when the meal is over and was enjoyable, a heartfelt Era delizioso! (it was delicious!) is the perfect way to thank the staff and end the experience on a high note.