You can add a custom field to the Accounts object to record details not readily available in the standard existing data entry options.
Navigate to the Accounts Object
Navigate to Setup | App Setup | Customize | Accounts | Fields.
Account Fields Page
Scroll down past Accounts Standard fields to the Accounts Custom Fields and Relationships section.
New Custom Account Field
Step 1: Choose the Field Type
Specify the type of information that the custom field will contain. Select the Data Type for the field to be added.
Step 2: Enter the Details
Custom fields marked as unique count against an object's limit of three external ID fields.
Step 3: Establish Field-Level Security
Select the profiles to which you want to grant edit access to this field via field-level security. The field will be hidden from all profiles if you do not add it to field-level security.
Step 4: Establish Field-Level Security
Select the page layouts that should include this field. The field will not appear on any pages if you do not select a layout. When finished, click Save if you are done or click Save & New to create more custom fields.
Account Detail Page: Default Field Location
The field will be added as the last field in the first 2-column section of these page layouts. To change the location of this field on the page, you will need to customize the page layout. To change the location of this field on the page, you will need to click Edit Layout.
Account Page Layout: Edit Page
Click and drag the new Account field and Drop it in another location. Alternatively, click Layout name to make changes to other page layouts as needed. Click Save to save changes to Layout.