when you go to the description page for a specific app there is a column down the left side.. where the price and purchase button is. Somewhere in that column will be a section labeled "Requirements". That section states which device(s) the app will work on. It will say something like "Compatible with iphone, ipod touch, and ipad" and then specify a minumum iOS version.
As noted above, just about any app that works on the iphone will work on the ipad touch with the exceptions noted above. The one to watch is to make sure that you don't select an ipad only app. Some apps have split and have two versions.. the ipad version and the iphone/ipod touch version, and there are some that are ipad only.
Another help is on the main itunes store app page there is a button at the top that selects iphone or ipad. For your purposes you want to select the iphone option.
Hope that Helps... and it will of course make more sense once you actually start using itunes.
Mary