I order to change the default Assigned To value for every newly created work item you will need to go to the admin of the system and select the project you wish to configure the default assigned to.
Open the project detail and click on the Process Flow tab and select there the work item you wish to configure.
From that tab you can select the value of the default Assigned To to one of the following options:
 

• Not Assigned – value to be used when you wish to indicate that the work items is not under anyone responsibility.
• Creator – The user who created the item
• Last Assigned – The user that was assigned before
• Specific User – name of a user registered in the system.