Question about T-mobile prepaid?

OK so I want to get the LG Optimus from T-mobile and use it on the 15$ a month unlimited texting + 10cents a minute plan. I'm just going to use wifi for web browsing. My question is if i leave my wifi are any facebook apps and weather apps going to auto connect to data and suck my account dry or is there a way i can turn off 3g so it only connects to wifi?
 
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