The spark plug boot is circled in green in the picture. An easy way to locate  the spark plug boot is by following the exhaust pipe to the underside of the scoot (when the scoot is on the kick stand (not the main stand). Remove the boot, and the plug should be visible. I would recommend cleaning your plug (wire brush) every 500-1000 miles or so, or at least when you change your  transmission oil. If you replace your plug, or want a cheap performance  boost ($7-$10) look for an iridium spark plug (50cc Vento (or clones) use NGK BR8HIX or Denso IWF24).  Iridium plugs cost 2-3 times as much as a regular plug, but there is a slight gain in performance when you use them. They have been dyno proven to  garnish HP and Torque gains in automobiles. The stock plug in a Vento 50cc (or clone 50cc) is a NGK BR8HSA but you can use a BR8HS of a BR8HSA is unavailable.

The below chart will help you “read” your plug and may help you determine if there are any issues with your fuel/air mixture or if you have a more serious problem.
  

Oil Fouled Oil Fouled Carbon Fouled Too Cold Too Cold Cold or Rich But OK Cold or Rich But OK Cold or Rich But OK Good Good Good Good Real Good The Best Best Best Best Good Good Good Kinda Hot But OK Hot or Lean But OK (?) Hot or Lean But OK (?) Hot or Lean But OK (?) Too Hot or Lean Pre-Ignition Range Too Hot or Lean Pre-Ignition Range Too Hot or Lean Pre-Ignition Range Too Hot or Lean Pre-Ignition Range     Too  Hot or Lean Pre-Ignition Range