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. If you are planning to trade altcoin then you should be really careful before invest in any of altcoins because full of shitcoins are everywhere.
Don't ever listen what people talking in trollbox (chat box in exchange website). Because most of them trying to promote their shit coins.
I advise you to watching price for some days maybe weeks before you get any position.
Good luck.