There is no Siri for iPhone for. Only 4S and above.
There are however several Apps in the Apps store that can do what Siri does. Here are some:
You can't. You need an iPhone 4S.
iOS 6: Which software features does my iPhone, iPad, or iPod touch support?
You posted in the iPad forum instead of the iPhone forum. To get answers to your question, next time post in the proper forum. See https://discussions.apple.com/index.jspa I'll request that Apple relocate your post.