Go to the settings menu on your iPhone and select the "Sounds" tab. Then tap "Ringtone." The ringtone you just created should be listed at the top under "Custom" or "Purchased."
Select the ringtone so a checkmark appears next to it. Now your new ringtone has been set as your default!
You can also assign different ringtones to each person in your address book. Learn how to set check out this article on assigning individual iPhone ringtones.