Flash a custom recovery. Most use either Amon RA or Clockworkmod (I use Clockwork, but it mostly comes down to preference). Go to xda-developers Nexus One forum for instructions on how to flash your recovery. Then you can flash a rom (ie- Cyanogen). Be careful though, while you can screw up a ROM flash with no permanent damage, screwing up a recovery flash can brick your Nexus. Make sure you follow instructions well.