There are several different things you can do.  First of all, there are several companies that make screening software that will screen out the majority of objectionable content.  Here are links to a couple of them.

Cybersitter
Netnanny
Cyberpatrol
Surfwatch

Each company's product does things a little bit differently.  Take a look and see if one of them might meet your needs.  There are also a few practical suggestions

#1: Keep the computer in a public place

#2: Make sure you are home whenever the internet is being used

#3: Don't let the computer save the password so you have to type it in every time.