There usually is an opening balance entry in the checking register.  That will be the balance on the prior bank statement (in your case the opening balance of the Dec 2016 statement).  If there wasn't an opening balance entered in the register it will be off so you have to create it when you first start.  You can of course add it now if it is missing.

Once it is added, click twice on the check mark in the register on that transaction.  This will mark it as reconciled.  Then begin reconciling with the Dec 2016 bank statement.  You may have to undo statements to go back.