A chart of accounts.

When you set up your accounting system, you need to make a list of the categories into which you want to collect data. This list is called "The chart of accounts"

The simplest chart would look like this:

Cash and Other Assets

Owners Equity and other Liabilities
(These first two items make up the balance sheet)

Sales Revenue

Direct cost of Sales

Fixed or overhead Costs
(These three items make up the main divisions of your Profit and Loss Account or P&L account)

It is most important that you clearly define these categories, and use them consistently. The reason is simple, these definitions will provide you with the measurements that will be the indicators that you use to run your business.