I am assuming that you have the Iphone SDK installed. If you need help with getting that installed or do not know what a SDK is then this blog will probably be over your head.
So first thing is to start Xcode. I have mind in the menu bar (sorry I am not a Mac person so I have no clue what it is called )
Once Xcode is started you will be greated with the opening screen that looks like this.
Once you are on the following screen click on File -> New Project and you will see the following screen
Make sure you are selected on the Iphone OS and Application option in the left hand panel. Also you want to Select the Navigation Based Application. Once that is done click Choose and then give a name to your project that has some meaning.
Once you do that you will see the following screen that has all the source code and build files in it.
From here you can click on the Build and Go icon at the top and it should compile correctly and launch into the Iphone Simulator where it will be a blank navigation.
There it is you have successfully created a very simple non-functional Iphone application.