Usually you trade on exchanges. I have several options I have tried. Huobi. Binance. Bittrex. Utopedia. They all good so far. Some have large delay for depositing and withdraw. For my experience, bittrex is the fastest. Binance is also good. Utopedia is slowest. Some have much more coins in exchange like bittrex. Utopedia can provide most recent new altcoins. hope this helps.