I am 6ft and riding a firefox kompac. My speed never crosses 18kmph
What is your "cadence" (number of pedal revolutions per minute)?  A healthy young cyclist should be able to "cruise" at 80-90 rpm, and that would be considered your top speed.  

If at 80 rpm you're only getting 18kmph (11mph) in your fastest gear then your bike gearing is quite low and you probably need to change something.  But many novice cyclists simply pedal too slowly.

With a multi-speed bike you should focus first on getting a "reasonable" cadence (maybe 60 rpm when "taking it easy", 80-90 when trying to go fast).  "Gear down" into an easier gear until you learn to maintain cadence, then slowly increase gear difficulty to gain strength and stamina.