Based on the 1.0GHz speed, your PowerBook G4 should have a DVI output on it. Which means that if your HDTV has an HDMI input, you just need to get a DVI-HDMI cable (readily available, not particularly expensive) to hook it up to your TV, and it should work fine. I've run exactly this setup with several different Macs.
For audio you're going to need a 1/8" jack (the headphone style) to RCA (red and white audio connectors), then an extension cable for one or the other to reach the audio input on your TV or home theater system. These should be dirt cheap at somewhere like RadioShack (though they've started pushing ridiculously overpriced "premium" cables recently, so maybe elsewhere). Hook those up, set your TV to use the correct audio and video input, and you should be good to go. (Oh, and don't forget to turn the volume on the Mac to max--this will give you the best quality, and you can then adjust properly with your TV or sound rig's volume control.)
Now, when you say "Can I even use it for apple TV?" I don't follow. You mean as a home theater hub in place of an Apple TV? If so, only sort of--it's going to be too slow to play most modern media files without skipping, so you're going to be limited to relatively small video files and DVDs.
