Bittrex and Poloniex is enough for me to trade bitcoin and also altcoins because it is on the mainstream where most of the traders here use that sites so you can easily sell and buy bitcoins and also altcoins without having any problem. Just register on that site and then deposit your fiat money and then you easily buy and sell different coins.
But Poloniex now is not that good as before, because of some issues happened a couple of months ago. Perhaps you knows what happened anyhow by the complain of their client in terms of transaction in withdrawal mate. In my case now, I often used bittrex it's a big exchange too like Poloniex, I've been using bitrrex for almost 1 year and so far I don't face any issues problem on this exchange.