To get an iPhone, you have three choices. If you want it fully supported and under warranty, you can get it direct from Apple (on-line or via an Apple Store), or direct from AT&T, and the price is the same from both. There may be a short wait to get one right now, until about the 15th of July, BUT, it is suggested if there is an Apple and/or AT&T store near you, go in and ask. Often pre-ordered phones are not picked up, and they may sell it to you. The third is to find a new/used one on the net (E-bay or similar) but I would not expect a great deal yet on new iPhone 4 models right now, a quick check shows typical prices for an iPhone 4 on E-Bay to be $700 to $800!!!!
The only "official" package and provider supporting the iPhone is from AT&T. You can Jailbreak the phone (as mentioned above), and move it to another GSM provider, but ONLY if you jailbreak it (a software process to unlock the phone) but be aware that Apple does consider this as a reason to void the warranty, so you may not get support if you do this.
There are a number of plans with AT&T, but things have changed recently. Also note that on AT&T, a data plan is MANDATORY with this phone for the entire length of the (2 year) contract. For one, AT&T no longer has an unlimited data plan. It is now $15 for EACH 200 MB usage (or part of) OR $25 for 2 GB usage and $20 for each additional or part of 2GB. Go over by 1 byte, you pay the additional charge. There is a $30/month additional fee of you want to use the phone as a Wifi hotspot, and you get no additional data in your plan for that $30/month. What a rip off!
SMS Text messages ARE NOT INCLUDED with an iPhone, but other messaging can be done under the data plan (like AOL, MSN, Skype, etc.) You have to buy SMS messages on your plan: 200 messages is $5/month. 1500 for $15, or unlimited for $20/month.
There are also Family Share plans and individual plans for minutes, and those vary by area, so you would have to check with AT&T to know how much in your area. Typically expect the contract to start at $45/month or more starting at around 500 minutes, roll-over is included.
Hope that helps with a bit more detail
