Decided to invest in Apple's latest and greatest handset? That's the easy part — now you need to decide how and where to buy it, and on what carrier. Here's the full rundown.

Can you get upgrade pricing?

If you're upgrading rather than signing up for a new line of service, the first thing you'll want to do is check to confirm whether you're eligible for upgrade pricing. To do that:

On AT&T, you can dial *639# to get a text message with your current upgrade status. You can also go here.

On Verizon, go here.

On Sprint, text the word "upgrade" to 1311 or go here.

On Cricket, it doesn't matter — there are no contracts. If you want an iPhone 5, you'll be paying full price (and Cricket hasn't yet announced what those prices will be).

Which carrier should you choose?

Good question, but also a very complex one. If you're looking to switch, take a look at Everything you need to know about buying a smartphone, which features a section on choosing a carrier.

In brief, you'll pay slightly less on Sprint than on Verizon and AT&T but at the detriment of LTE coverage and 3G data speed; Cricket will be the cheapest, but you'll pay much more for the phone upfront since there's no contract involved. Keep in mind AT&T's FaceTime controversy, which requires that you be signed up for one of the carrier's new Mobile Share plans in order to use the video calling service over cellular — neither Verizon nor Sprint are restricting it. Then again, only the AT&T model will support simultaneous voice over data, so that's something to think about as well.

You may also want to consider data buckets: Verizon and AT&T no longer offer unlimited plans (unless you're grandfathered in), whereas Sprint remains both unlimited and unthrottled. Cricket's plans are unlimited, though they throttle down to a crawl once you've hit a limit ranging from 1GB to 5GB depending on your specific plan.

Speaking of grandfathered unlimited data plans on AT&T and Verizon, you need to be careful. On AT&T, you can't "upgrade" to a Mobile Share plan if you want to keep unlimited data, which means you won't be able to use FaceTime over cellular. On Verizon, the only way to keep your plan is to stay away from subsidized phones, so you'll need to pay full price — $649 or more — for your iPhone.

How much will it cost?

On AT&T, Verizon, and Sprint, you'll pay $199 for the 16GB model, $299 for the 32GB, and $399 for the 64GB on a new contract or extension (if you're eligible). Without a contract, you'll pay $649, $749, and $849, respectively — a $450 premium.

Where can you buy it?

The online Apple Store begins accepting pre-orders at 12:01AM Pacific Time on September 14th, which is 3:01AM Eastern Time. You can also pre-order from your carrier of choice at the exact same time — AT&T, Verizon, or Sprint. Cricket hasn't announced a pre-order program, but it's also not launching on the same day as the others; it comes a week later on the 28th.

You'll also be able to try your hand at walking into a store on launch day, September 21st. Apple Stores will be open at 8AM local time, but naturally, you'll want to get there very early if you want to be assured a phone and don't want to spend forever in a line that wraps around the building. Some carrier stores may open early on the 21st as well, but you'll want to call ahead to find out details from your location of choice.

Finally, the usual host of third-party retailers will be offering iPhone pre-orders and launch day sales, including Best Buy, RadioShack, and Walmart. Notably, Walmart will offer it at a $9.03 discount, taking $25 pre-orders at 8AM on September 14th and selling units at the same time the following week, September 21st.