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. YOu can't wait till the price of bitcoin will decrease if you are going to wait and then buy bitcoin it is better to buy bitcoin now and start trading because they are potential that bitcoin price may increase by itself and you can start tradings now with altcoins go to matket place (altcoin) for more information about icos and altcoins there.