Its not the APK file that your iPhone app can generate. Its actually called as a build file (.app) that is created using the Xcode IDE./ .ipa file.
The following points gives a rough overview about the whole process.
Before creating this .app file certificates and provisioning files should be obtained from the Apple developers a/c ($99)
The certificates and provisioning files are used for security reasons to denote that this application is not a hacked one and to make sure its been developed using apple supported technologies only, Its like a digital signature.
This provisioing file has to be added with the IDE for that particular project by double clicking the file once downloaded from the developer's site.
Each provisioning file will be interlinked with a bundle identifier which is also created using apple developer's account
After adding the above mentioned provisioning file, you should add the bundle identfier of that provisioning file into the "Bundle Identifier" key that can be found in Targets -> Summary -> iOS Application target
Only after adding this you can integrate the provisioning file to the project (Matching the bundle ID to the appropriate provisioning file).
After adding this just issue CMND + B in order to build the application which will create the .app file
This .app can be found under "Products" folder that is listed out under the "Projects Navigator" section.
Hope, this would be little risky for you to follow up, since you are newbie to iOS and the above points would give you an overall idea about the .app file in iOS.
Cheers !