I'm pretty new to development, and have modified code found elswhere on the internet.  The code is working find in my sandbox, but when i attempt to deploy it to production, there's no test classes and so it fails.

Here's the code:

// Use a name besides "lead" otherwise you'll get compilation problems due to symbol collision
public with sharing class leadApexZipCode {
    
    private static boolean isTrigger = false;        // Pitfall #4: Used to prevent infinite loops
    public class leadException extends Exception {}

    /* format from ziptasticapi.com:  
        {"country":"US","state":"CA","city":"SAN JOSE"}
    */
    // Format returned by ziptastic API
    public class ziptasticReturn {
        string country;
        string state;
        string city;
    }    
     
    // Trigger Handler to do zipCodeLookup when new records are created
    //  Do this after the record is created so we have an ID we can reference
    public static void onAfterInsert( List<lead> triggerNew)
    {
        // Pitfall #4 - Prevent infinite loops
        if (isTrigger == true) {
            return;                // Just return if this is called as a result of our DML operation
        } else
           isTrigger = true;    // Set this so we know we caused ourselves to be called again
        
        // Must pass IDs & not SObjects to @future because the SObject may change by the time @future executes
        List<Id> leadsId = new List<Id>();
        for (lead a : triggerNew) {
            leadsId.add(a.Id);
        }    
        system.debug(LoggingLevel.Info, 'onAfterInsert called with '+triggerNew+'; sending IDs='+leadsId);
        makeZipCallout(leadsId);
    }
    
    // Pitfall #1 & #2 - Triggers must do callouts from an '@future' and (callout = true) anotated function
    @future (callout = true) 
    private static void makeZipCallout(List<Id>acct)
    {
        system.debug(LoggingLevel.Info, 'makeZipCallout with '+acct);
        string resp;
        
        // Pitfall #3 - Fetch records of the IDs for updating
        List<lead> leadSet = [SELECT Id, Postalcode, City, State, Country
                                    FROM lead 
                                    WHERE Id = :acct];
        
        for (lead a : leadSet) {
            
            // Note this version of the API is only for the US
            string endpoint ='http://ziptasticapi.com/';
            endpoint = endpoint + a.Postalcode;
            system.debug(LoggingLevel.Info,'zipCode.cls: calling endpoint='+endpoint);
     
            HttpRequest req = new HttpRequest();
            HttpResponse res = new HttpResponse();
            Http http = new Http();
            req.setMethod('GET');
            req.setEndpoint(endpoint);
     
            try {
              res = http.send(req);
              if (res.getStatusCode() != 200) {
                throw new leadException(res.getStatus());
              }
            } catch (leadException e) {
              system.debug(LoggingLevel.Error, 'Error HTTP response code = '+res.getStatusCode()+'; calling '+endpoint );
              return;
            }
      
            resp = res.getBody();
            JSONParser parser = JSON.createParser(resp);
            parser.nextToken(); // Start object "{"
        
            ziptasticReturn zipInfo = new ziptasticReturn();
            // This convenient method reads the JSON stream into a class in one go
            zipInfo = (ziptasticReturn) parser.readValueAs(ziptasticReturn.class);
            a.State = zipInfo.state;
            a.City = zipInfo.city;
            a.Country = zipInfo.country;
            
        }    
        update leadSet;        // Call DML operation to update
        isTrigger = false;        // Pitfall #4 - Trigger is done so reset this
    }        // makeZipCallout()

}