Buy low, sell high. You might want to go find a guide on stocks, since trading crypto is exactly like trading stocks.
Okay.We can see price difference at exchanges.So if I get a bitcoin for low price at one exchange and try to sell it for high at another exchange,why would people but it from me?They can get it from the very exchange I got for low price.
You are talking about arbitrage, i.e. taking advantage of the price difference by buying / selling at two different exchanges and profiting from the differentials.
The problem is that crypro currency trading is still a very illiquid market in fiat terms, So withdrawing dollar from one exchange and placing them in a second is time consuming and expensive.