Disabling 3G

Okay, so yesterday my parents got my phone bill. Supposedly I exceeded my plan or something which ended up being a bit expensive. The thing is, it wasn't my fault. On the bill it said something about my internet usage, or 'packet data' being over the limit. I've only ever used WLAN or so I thought. Being paranoid, I turned off my phone to prevent anymore internet usage without me knowing till I found out the cause of the problem. Anyway, this morning I woke up being peeved because stupid Nokia decided to make us spend more $. I think I found the problem- 3G. The stupid thing has been draining my batteries and using the internet without my knowledge!! Grrr. So anyway, I think I've managed to delete it after one hour of searching on google with no luck. Now I'm just waiting for the next phone bill. (Oh and my sis racked up an even higher bill than me- about double I think? No idea what her problem is though)