C# Restaurant Bill Calculator Help

[ImMalkah]

Ok, so I have this practice problem I don't know how to do. Here's what my UI looks like.

​

My problem is that I need to use variables to do this. So when I press calculate bill, it will use those variables to determine all the things on the left hand side. So the appetizer total, mains total, refreshments total, subtotal, tax and the final total.

I WILL REP WHOEVER HELPS ME BECAUSE THIS IS A VERY IMPORTANT ASSIGNMENT.