How to order half amount of food in French restaurant, bistro, cafe, etc.?

Hello,

I'm going to France next week. This is the first time for me to go to France and I have a question about how to order half or small amount of foods in restaurants, bistros, or cafes. I heard that they sometimes serve you a huge amount of foods and you cannot finish all of them. Actually, I'm not able to eat a lot… So I think I need to learn before going to France. Could you teach me a couple of typical expressions when you want to order a half or small amount of foods?

Also, could you teach me a couple of typical expressions when you want to share your foods with your fellows? If they don't have an option to give us a small amount of foods, I want to share my foods with my friends. For example, suppose you go to a restaurant or bistro and order something a huge amount to share it with your friends, families, etc. In this case, how do you ask waiters as you can share the food? Thanks a lot in advance!

Andy