What is the best type of meal to order in a restaurant when you're on a diet?

I am going to Bella Italia tonight with a friend, which is a very nice Italian restaurant. However, I am on a diet, so I don't want to blow this by eating the wrong things. I have had a look at the menu and apparently eating half a pizza (margarita - cheese and tomato) has about 390 calories. Which is fine, but is there anything healthier I could choose? I am a vegetarian by the way, and the ceasar salad seems to have a million different dressings on it! Help would be greatly appreciated, I don't want any higher than 500 calories in my meal :) thankyou!