You had a great idea and used what you learned about X-Code and Objective-C to build it. You've tested it extensively on the iPhone Simulator (one of the components of the iOS SDK) and now it's time to test it on an actual device.
Why, you might ask? There are two primary reasons.
1) The iPhone simulator uses the memory and processor of your Mac. Yeah, that makes it a SUPER iPhone. So, while your app might run nicely on the simulator, you might find that it crashes on a device since it doesn't have the memory resources available when it was running on your computer. You MUST know if it works on a device. So test it there.
2) The iPhone simulator can simulate some movement, but it cannot do put the accelerometer to any real test. For that you need a device.
So what's the play?
It's now time to join the Apple Developer's Program. I imagine you're thinking, I'm already an Apple developer; I have an account and everything. Yeah, but this is different. You need to go to the iOS Developer Program website and enroll for $99.00 (click the "enroll" button). Yup, you have to pay now and it's a cold, hard bummer. I know it's tempting to complain, but just remember that everything up to this point has been free: the SDK, all the resources and all the research materials. Time to cough up. And just think, if you make $100.00 selling your app (not at all unheard of), its paid for itself.
Because you DO have to pay before you can test your app on a device (and all the instructions on how to do this are clearly indicated on the site once you enroll), I recommend that you wait until you have completed and tested your app on the simulator before you join. Why pay for a year and let the days dwindle by while you write code?
After you have tested extensively (and you should try it both the iPhone 3G, the iPhone 4 AND an iPod touch if possible) you'll need to submit it to the app store. Again, all the instructions for this are on the site once you enroll.
Happy apping and good luck.