There are a number of things you will need to address, so one option is to issue an interim letter simply accepting the resignation, thanking him/her for their service and indicating that you will write again with the necessary arrangements.
As Mrs A has said, ACAS is a good starting point. Others will add to this list, but you will need to cover last day, accumulated holiday, final payment of salary, P45, pension details if appropriate, return of equipment, keys, records, passwords etc etc