You have to pay for that on your bill. for it to work automatically. But theres a downloadable cab that allows you to do it for free. its in the tweaks hacks tips and modifications thread in the regular tp2 forum. theres also one that allows you turn you tp2 into a wifi network.
http://forum.ppcgeeks.com/showthread.php?t=78019
^^^ theres the link its the first cab on the list enabling tethering stealth mode.
Edit:
PS also when you dl the cab you have to access the internet sharing program itself instead of allowing it when you connect your phone through usb. and the network COnnection has to be sprint and not phone as modem because it will not work as phone as modem unless your paying for it with sprint