Get Started with Apex Triggers

trigger AccountAddressTrigger on Account (before insert,before update) {
    for(Account acct:Trigger.New){
        if(acct.Match_Billing_Address__c=true && aacct.BillingPostalCode!=null){
            acct.shippingPostalCode=acct.BillingPostalCode;
        }
    }
}