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.
i agree with you that no one have knowledge that when is bitcoin increasing and decreasing its price and i think its depend on us if buy bitcoin then price go to high and when sell our bitcoin then price go to fall so simply bitcoin price is depend on adoption of bitcoin.