April 1 is the date of your first withdrawal the year after you turn 70 1/2. After the first year, your annual RMD must be withdrawn by 12/31. If you are still working and contributing to a 401k, you do not need to take withdrawals from the 401k until you do retire.

If you go to www.irs.gov and look for publication 590 you will find the withdrawal tables to be used.