Question:
How to pupolate a field with the value of another field whenever a new record is created for the first time?
Answer:
Use Post Default Value property of the field using experssion to populate its value with another field value.
Example:
To populate value of field “Sub Status” with value in field “Status” I would use below given expression in Post Default value of “Sub Status” Field
Field: 'Status'
You can read more details about Post and Pre Default Properties from the link given below
http://siebelunleashed.com/pre-default-and-post-default-properties/