How to measure handlebar? pls
You need to measure the grip area diameter (the diameter of the handle bars where the brake levers clamp onto). I'd suggest using a pair of calipers. 

22.2 mm will fit flat bar (typically mountain bike/flat bar road) components, 23.8 mm will fit drop bar (typically road) components.

Note that you need to also get compatible pull brake levers -- in your case, you have a caliper brake so you need a short pull brake lever (ie. one that is marked for not linear pull/V-brakes).