Will They Charge Me For Imessage?

So I'm Getting An Iphone 4 Soon And Like My Sister Has An Ipod So If I Imessage Her When I'm In School And I'm With 3G Network Will They Charge Me In AT&T 0.20 For Each Message Or They Won't Since Its Imessage Or Should I Just Connect To The Wi-Fi In My School? Someone Please Because I Don't Really Want To Get Charged Just For Sending Texts. Whoever Tell Me The Most Information And Is Right Will Get The ** Best Answer***

I have an iPod and I don't get charged when I message my mum and uncle