How to solve UML Class diagram

For my modelling class I have some homework involving creating a UML class diagram which I'm quite troubled with. I'm having difficulty identifying what is the class, the attributes, and the functions.

Here's the information that was provided to me:

The document header in a sales order includes data that are valid for the entire sales order. Examples are customer data such as partner functions and customer PO number, dates, and order total. Each sales document can include one or more sales document items, which contain data about each item included in the sales order. Examples of item data are material number, description, and quantity. Each item can be associated with a different item category, such as standard item, text item, and free-of-charge item, which determines how the item is handled with regard to pricing, billing, and shipping. For example, there is no charge for free-of-charge items. Finally, each document item can include one or more schedule lines, which specify delivery quantities and dates. The sales order corresponds to an invoice document.

Any help appreciated, Cheers!