Iphone with no data plan? what to turn off so i won't be charged? 10 POINTS!?

i have an iphone 3g i think it is with no data plan. i just took the sim card from my old phone and put it into the iphone and i was just wondering, is there any settings or anything that i need to turn off so that i won't be charged for anything other than my regular plan stuff? and does anyone know what internet tethering is? is that free because i can't turn that off i think. please help thanks.