Start by educating yourself on the Net and child safety issues. While filtering and blocking tools like NetNanny and ChatNanny are not fully effective, there are sites that offer useful tips for parents-try www.safekids.com, www.getnetwise.org or www. netparenting.com for starters.
But ultimately, the real controls lie offline. The rules are a common sense extension of the classic "dont talk to strangers" dictum. To begin with, share an e-mail account with your child (but dont abuse his or her privacy). Make sure that your child understands the dangers of offering e-mail ID, phone numbers, school or home address and real name. Do monitor Net time-and ensure that your kids dont get on the computer when neither parent is around. Dont breathe down their necks, but do monitor chat sessions. In the end, this is like the real world-there are no guarantees.