Well i wonder which company is it, surely not godaddy or namecheap.
well sorting your issues, check in the control panel for "URL Forwarding" or maybe "url Masking", atleast this is what i see in godaddy, not sure about your provider.
If you cant find it, here is another solution.
Make a new notepad file & name it as index.php
copy the following in it
<?php
header("Location: http://example.com/");
exit();
?>
Where example.com is the domain on which u wanna reach ultimately. (myfirstnewdomain.com in your case)
Now open an account on some free hosting site (i prefer 110mb.com, no am not connected with them)
point your nameservers of mynewdomain.com to the 110mb servers (mostly will be ns1.110mb.com & ns2.110mb.com, but please check it yourself, it'll be on the left side once u login into your account)
ftp this index.php file into the root directory of your account, now after 48 hours go to mynewdomain.com and you'll get directed to mynewdomain.com.
P.S. - now you'll ask why wait 48 hours?
look, when u change nameservers the effect doesnt reach on ur isp instantly, it takes 24-48 hours to propagate correctly, so just have paitence

and yeah there is no charges for opening an account at 110mb

Still confused?
drop me a pm