Text messaging is available as a standalone product and as a part of any combination of US Mobile services. You are billed for both incoming and outgoing text messages.
Note that at anytime if you feel that you are running low on texts, you can top-up via the user dashboard. Also, text messaging can be disabled within the Settings tab on your phone.
Since iMessages use data they are counted to your data limit and not your text messages. If you are looking for information on how to configure MMS or your APN Settings please consult the respective links.