How do you make Google homepage on Nexus?

If you are referring to google Now, it should already be available to any Nexus device that has Android 4.4 OS or above. For the Galaxy Nexus, if you are able to flash a custom ROM that has Android 4.4, you can then install the google Now launcher apk, which is avaiable on the web.

If you mean mimicking the google homepage, similar to the desktop, you can simply find a wallpaper that has that similar look. The search bar you could find an app that can move it from its fixed position at the top of the screen to the middle.