In order for you to select the right category (i.e. expense account) for each of your documents, you must first import your chart of accounts from your accounting system. This requires you to set up an integration between with your company's accounting system. In the integration setup, you will have the option to import your chart of accounts. 

With Workflows, you can automatically assign categories to your documents. Do this by setting certain document traits as triggers for specific categorization actions. You can read more about how to create workflows here.

You can always manually select a category/account for a document by clicking on the Category drop-down menu on a document. Simply select the desired category from the menu:

As soon as you manually select a category for a document, that same category will be automatically selected for any future documents from the same vendor - unless you have set up a workflow that specifies otherwise, of course.