Can anyone help me how to know if the bitcoin price will decrease?
and how to trade bitcoins and earn? 
Because i invest bitcoins in yobit.net but i didn't know how to properly trade bitcoins. The price of bitcoin is very unpredictable. Also not all member in this forum is not psychic that can predict it actually. But you can do trade anytime you want. Aside from that for you to know the proper way of trading just observe the traders in the different exchange platform.
why do you think you need to predict the future with 100% accuracy and be a psychic to do it?!!
have you ever heard of the term speculation and technical analysis?
these are all ways that you as a trader can use to "predict" the future price but with some room for error and for that error there is also some ways to make it less risky.