See the RAMCO community for detailed video instructions.
Here we will provide a very basic overview of how to accomplish this on your system.
1. Navigate to Settings > Customizations > Customize the System
2. In the left side menu click on the arrow beside entities to view all entities available to you.
3. Select the arrow next to the Entity you wish to Customize and then click Fields
It may take a few moments for the field list to load. Once it does you can click New at the top of the list to create a new field.
4. Give your field a Display Name and then click anywhere else on the form to automatically fill in the Name field.
5. Leave the Requirement Level at No Constraint and Auditing as Enabled as the Defaults.
6. Select the Type of Field
For more information on the types of fields this blog post from Hitachi Solutions gives a good description of each. http://blog.customereffective.com/blog/2013/05/data-types-available-with-microsoft-dynamics-crm-2011.html