The rules is very simple . Buy low , sell high . Also when you bought a coin for a price , and that coin is going down , you should wait until is going back at the original price and sell that coin 
Yeah that's right, its simple buy low and sell high, but be careful to buy a shitcoin because your money really can be wasted.When you buy coin and the price is go down you should hold it and wait until the price is increase.