Hi! This is a great test for you, you can do it!
First, if this is a well-known restaurant... I would suggest going onto their website, looking at the menu, and then deciding beforehand what you will be ordering. It's SO easy to get caught up in all of the unhealthy options if you don't plan ahead.
Stick with what you decide beforehand. Also, a light soup before your main course will help to keep you from over-eating. Remember to look out for words like: fried, creamy, batter. Avoid these like the plague!
You're much better off with: steamed, grilled, light...
Also, don't be afraid to make special requests (dressing on the side, broccoli instead of french fries, etc...)
I've also noticed that the person doing the most talking always ends up eating less/slower. Be a chatter box and get into your stories... you'll be talking more than eating which is really what social gatherings are all about anyway!
Good luck!