For trading there are many places to do it. Poloniex, Bittrex, Bitfinex. Those ARE NOT a safe place to keep your currencies, at least you will be trading . if you want to buy BTC you can do it in bitstamp, localbitcoins. To buy there you need a wallet to keep your btc safe, like blockchain.com. So, yo buy your btc in bitstamp or localbitcoins, then you transfer to poloniex to convert from BTC to dogecoin or litecoin per example, and then you move the LTC o DOGE to a secure wallet or to speculate about the fluctuation prices of the cryptos and buy an sell -let me tell something about it, in cryptocurrencies, almost every trade is a high frecuency trade, sometimes in seconds or miliseconds, it's hard and exhausting and there are not regulations, sometimes is a bloodbath there, hardest than any other market. For steem you dont need to do that, you can buy in bitstamp or localbitcoins, and then directly convert to steem in this place.