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. No one knows when it will increase or decrease, it depends on the market.
If u want to earn with trading, it has no point when u trade with 0.1 btc or something. I advise u to buy and sell btc with a margin of 5 per bitcoin. So each btc u sell u get 5 above the btc price and then u buy 5 below the price, so u make around 10 per traded bitcoin, which is not bad.
Don't buy and sell worthless altcoins, because yobit supports many shitcoins and doesn't care about it's users.