Can i bring lots of foods in cans like sardines, mineral water etc in my checked in luggage?

What kind of food that need declaration? And how to declare?

If i booked for 20kg luggage allowance n let say the bag itself weigh 2kg, it means that i only have 18kg balance for my things right?

This is my first time going overseas. Going to japan this march 2014 with AA flight.