Customizing the Salesforce.com database can be done within the Setup menu.  This is found in the upper right controls.

Customizing Salesforce.com Fields

The Setup area is divided by native data objects – Contacts, Accounts, Opportunities.  To add a field, be sure to select the respective data object to add a field or modify a field within.  Thus, for a new Contact field, select on the left, the menu item for Field (Customize>Contacts>Field) under the Contacts area: 

Next, you can add a new field by selecting the New button on the screen: 

A wizard will open to prompt you for the following:

1. Field Type:  This is the kind of field you wish to insert.  A picklist, text field or checkbox are examples.  It is important to understand the ramifications of the type of field you select.  For example, if you want users to have specific values to avoid errors in reporting, be sure to use a picklist versus a text field.  A calculated field may make sense if there is a dependency on other values.  The key is to think systemically – what are the impacts across the system.  It is not enough to be merely pragmatic.  Your data, users and system have a multiplier affect on the field type decisions  you make.
2. Field Details:  Enter the name of the field.  Note that this autofills the field name.  This name is used across the system as an identifier for use in email templates, calculations, filtered lists and an array of other functions.  If you change this name later, be sure to understand the impacts across the system.  If you are entering any field values for a picklist, be sure to order them accordingly.  Use the Help Text to give a floating indicator of the field for users and the Description for remembering your purpose for the Field information.
3. Page Layouts:  If you are in Salesforce.com Enterprise edition, this will have relevance.  Select the page layout view in which this field will appear.  Otherwise, you can keep the default page layout.  After you are done saving, you must move the field to the respective section in the layout: 

Focus on Salesforce.com Systemic Impacts

Your Salesforce.com can have as many fields as you like.  The true art is having as least amount of fields as possible.  Every field added creates a ripple effect in:

• Added work for your users
• Data management
• Reporting impacts
• Communication template designs
• Page layout management

Be sure the fields are useful.  Their benefit should outweigh the costs, otherwise adoption and management of your system may be affected.

Hope this helps. Or you can get further Salesforce consulting for customizing your system.