As above, use the Remote app.
Once you have it, turn on Home Sharing on your iPad and AppleTV (and your Mac/PC with iTunes if you have one in the house) and you have a great home media solution.
- Use the Remote app, select your AppleTV, and you can play music or videos on your TV.
- Use the Remote app, select your iTunes library and you can play music on your home PC/Mac. (Or, if you use the volume control, you can set it to play on your AppleTV. The nice thing about doing it this way, is you get a volume control for your AppleTV, which you don't normally get).
- Or, via Airplay. Turn on Airplay on the AppleTV, and then (most times) when you're viewing an image or playing a video on your iPad, there's an Airplay icon which will enable you to output to the TV.