What to do if carbon monoxide is present?
The first thing you're going to want to do is get fresh air into the cabin, by ensuring the vents/windows are open. This will dilute the amount of CO in the cabin and stop your symptoms getting worse - allowing you to land as soon as practical.

There is also some good information online which contains further advice:


  
  Turn the cabin heat fully off.  
  Select maximum rate of fresh air ventilation to the cabin.  
  Open windows if the environment, flight profile and operating manual permit.  
  Consider using supplemental oxygen if available and if doing so would not  introduce another safety or fire hazard.  
  Land as soon as possible/practicable.  
  Inform Air Traffic Control of your concerns and intentions.  
  Select a leaner fuel mixture if possible.  
  After landing seek medical attention as soon as possible.   
  Before continuing the flight, have the aircraft inspected by a certified mechanic  
  


If you're wondering about the first point, the reason is that the cabin heat is the primary way CO enters the cabin of light aircraft. 


  Typically most piston powered aircraft obtain their cabin heating by directing fresh (ram) air over the engine muffler (silencer). If there are any cracks, holes or poorly fitting components in the exhaust system, then CO-rich exhaust gases can enter the cabin. Engine exhaust may also enter the cabin through inadequately sealed firewalls and wheel wells etc.