I don't have a data contract with t-mobile. I use wi-fi for most of my needs. But here is what is interesting. I have unlimited text and MMS. But my MMS only works with mobile data on.
So, what I do is I turn on mobile data at home or work. There, my wifi handles the data that gmail, chrome, facebook, etc use. And I can receive MMS via mobile data. I get no data charges this way as mobile data is being used for MMS only. Then when I leave home I turn off mobile data. If I leave mobile data on when I leave or turn it on while away from wifi, the gmail, etc programs will use the mobile data and I get charged. (All of this turning on and off for my benefit is done via tasker settings I created.)
Of course your contract with your carrier will differ on how data is handled, etc..
What Harry2 is referring to above is your APN settings. Your carrier will have these for you.
In Jelly Bean: Settings > More > Mobile Networks > Access Point Names