I have created the custom tab in manage customer section in Magento 1.9. Now I have to create the attribute in my custom tab but don't have any idea how can I manage the attribute please suggest.
There are a number of free extensions that can be used to create customer attributes. I use Zestard Customer Attributes, which is primarily focused on adding fields to the customer registration form, but would likely accomplish your goal of creating a customer attribute for use elsewhere. Otherwise, you may want to take a look at These alternatives.
Hope this helps!