I need help on how to make my bike faster
There's a nice gear calculator at Sheldon's site. I did a quick calculation for you, and at 50/12, at 90 RPM, you should be going 47.4 km/h.  If you upgraded your front chainring to a 54, and your back cassette to go down to 11, at the same RPM you could go 55.8 km/h, which is a significant increase.  You can play around with different options on the calculator to see how much it would help to get different gearing combinations.

You might find that you won't get much extra speed out of bigger gears as your bike has a somewhat upright posture and therefore you'll be quite limited in what kind of speed you can get.  I think that 50 km/h is plenty fast for a bike and that you should just focus more on going faster on the flats and uphills.