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. volume transaction in yobit is good only pair altcoin
but you want trading bitcoin pair fiat money is good huobi btc-e
earn trading only buy in low price and sell in high price, if you want trading altcoin you can survey about coin, in coinmarketcap dot com
for beginners buy low and sell high will applied but knowing the movement by tracing it before buying because timing is also important.