You need to get a single-speed freewheel:
http://www.retro-gression.com/produc...mx30-freewheel
Then you need to move spacers from the drive side to the non-drive side and re-dish your wheel, which you can read about towards the bottom of this page:
http://sheldonbrown.com/deakins/how-...onversion.html
However, re-dishing a wheel will generally result in a weaker wheel with very uneven spoke tension.