http://www.my-gti.com/category/in-car-entertainment/rns-510-in-car-entertainment
You'll need the antenna/aerial adapter for a start.
Upgrading 2nd generation radios (RCD 300, RCD 500) to 3rd generation (RCD 210, RCD 310, RNS 310, RNS 510, RCD 510) antenna adapter
Suitable GPS Antenna that will come with the unit.
You'll need to code the unit using Vagcom/VCDS and also change the outputs to low if you have the factory amp installed.