Question: How can i make my application start on the iPhone startup?

Answer: 
If you plan on distributing your app via the App Store, this is not possible.

As Randolfo mentions, many things are possible via jailbroken installations.
Only some very specific background applications can be started right from the iphone startup.
This includes VOiP applications for instance:

*"Including the voip value in the UIBackgroundModes key lets the system know that it should allow the application to run in the background as needed to manage its network sockets. **An application with this key is also relaunched in the background immediately after system boot** to ensure that the VoIP services are always available."*

As taken from:

<http://developer.apple.com/library/ios/documentation/iPhone/Conceptual/iPhoneOSProgrammingGuide/BackgroundExecution/BackgroundExecution.html#//apple_ref/doc/uid/TP40007072-CH5-SW15>