There are many Bitcoin, and Altcoin Exchanges. Knowing which to use and how to use them can be difficult to navigate. This is an area you either want to research or get good solid advice in. Exchanges come and go all the time.
The top 10 listed on coinmarketcap.com are Bitfinex, bitFlyer, HitBTC, Bithumb, Poloniex, Bitstamp, BTCC, GDAX, Binance, and Bittrex. You can trade all kinds of Altcoins, Bitcoins, and other currencies at these exchanges.