what you talkin bout Willis??
All modern televisions have a coaxial connection. This is the connection that you would normally screw like a cable from cable tv onto. Your tv most certainly has one.
Then you just need an adapter for your Atari. They are cheap, like $2.
http://cgi.ebay.com/NICKEL-RF-ADAPTE...QQcmdZViewItem
Then you are all set. I would not bother with an AV mod right away. The adapter works pretty well and gives a good video quality. Plus it is much cheaper and no modding required.