want to create differential job with 30 days overwrite protection only

Hello Everyone,

i am using symentac backup exec 2012. in this i want to create a Differential job in which:

1. data remains only for 30 days.

2. after 30 days. on the 31st day which is actually the 1st day of cycle, data of previous 1st days should be delete.

 finally i want only 30 days data and after the 30 days, data overwrite with daily basis.

and please suggest which type of media set i need to be created.