Write a trigger to automatically populate Account shipping address fields with billing address fields if Copy_Billing_To_Shipping__c checkbox is checked.

trigger AccountTrigger on Account (before insert, before update) {
    for (Account a : Trigger.new) {
        if (a.Copy_Billing_To_Shipping__c) {
            a.ShippingStreet = a.BillingStreet;
            a.ShippingCity = a.BillingCity;
            a.ShippingState = a.BillingState;
            a.ShippingPostalCode = a.BillingPostalCode;
            a.ShippingCountry = a.BillingCountry;
        }
    }
}