Wifi is default.
Except for carrier's data requests (not charged to your data plan), and multimedia messages (that do count towards your data) will always be over your 3G/H+/LTE data signal.
My Data Manager Free, or similar app, will help you track your data usage and you usually have an option to shut all data communications off (I'm not sure about stock though, may need root)...if you do turn data off, you won't get your MMSs until you turn data back on.