I suppose all you need (assuming you already have a corny setup) is
A mixed gas cylinder.
A mixed gas regulator.
Correct diameter line to go from your corny to your guinness tap (i dont think its the same diameter as is needed for the taps norm sells)
The problem you will have is getting the mixed gas cylinder - dont know where you can buy the gear though.
I have a mixed gas cylinder - but no regulator. I got it from someone i know in the pub trade.
I think i may have heard that sometimes nitrogen is not strictly necessary, aerating the beer on exit from the tap may be enough to give it a bit of creaminess without nitrogen - I know the guinness tap has a little disc with holes in it under the spout, that the beer has to pass through. This may go some way to doing what im trying to describe.
Edit: just saw that you said you had nothing except the tap. There is plenty of info on here about buying corny setups. All you need to remember that if you are using mixed gas, you need a different regulator.
