I am currently doing my final year project such that, I need to make an Android phone (Nexus S) communicate with an Arduino Uno such that the phone is an NFC tag.
I figured out that an NFC shield might be a way for them to communicate. However, after I searched around, I have no idea how to start writing the code. How can I do it?
So in short: Nope, you can't do this unless you are willing to invest several month of research and development.
Technically the NFC chip in the Nexus is capable to do this, but the public Android API does not expose this feature and it is not trivial to enable/implement this.