It should at least have 4 types of food: carbs (carbohydrates), proteins, vegetables, and fruits. Be conscious about what types of foods to put in bento box when you are packing lunch. My children’s bento ratio is 4 : 2 : 1 : 1 (4 parts carbs : 2 parts protein : 1 part vegetables : 1 part fruits). I sometimes pack 2 parts of veggies inside the bento (skip fruit section) and prepare a separate container just for fruits.
Buying bento gear can be loads of fun, but if you're just jumping on the bento wagon and need some supplies to get started you'll probably need a list of the basic essentials so you don't break the bank on your first shopping spree.