Understanding Object Usage

Hello All,
This might not be the best place to post this, but since at least some of
you probably come from a procedural programming background you might be able
to help me understand this.

I have been working with Java for a little while now and I'm starting to use
it more and more. I have a slight disconnect when it comes to understanding
a few things.
1. When to create an Object? (e.g. a Customer.class would probably have a
Address.class)
2. When to use an interface over just using a class?

Take for example a customer order. A normal customer order would consist of
a header and a detail.
So, would you create a header class and a detail class, then an order class
that consists of a header and an array of detail? Or do you just create an
order class that would have header information and detail arrays?

The header class would probably have a customer class, which probably has an
address class.
Now would have an item number, description, quantity ordered, quantity
shipped, and price. But would the item number be better as an
ItemNumber.class that contains the description?

Basically, I'm just looking for where do you draw the line in creating
objects?

I have read several books, but all the examples are usually pretty small and
don't really seem to go too deep into this kind of stuff. So any links to
good articles, books, or just advice in general would be appreciated.

--
James R. Perkins