Planning on getting iPhone 4 but i have a few noob questions.

you can buy the iPhone with no-commitment, but a data plan is required.

You can place the iPhone is wifi mode, but seriously, if you can't afford the data plan, you shouldn't get getting an iPhone at all.

 

Like was said above, AT&T requires you to have a data plan, even if you're not under contract. You can usually get away with it for a couple months, but they'll notice eventually and force you to put a data plan on. It can just be the $15/month one that has very limited data requirements. You can turn Cellular Data off in the new iOS 4 software, so you won't go over on your data at all.

Apps access data the same way applications do. If your iPhone is connected to a wi-fi network, it will use that. Otherwise, it uses the Cellular Data (unless, of course, your Cellular Data is off).

 

THIS.