I'm already familiar with coinbase and Gdax but they only cover four coins at the moment.
What is a good exchange where I could buy and sell alt coins like Ripple, Dash, Monero, and Cardano?
Ideally, the fees will be zero or very low. Obviously I want something secure as well.
The easiest way is visit
https://coinmarketcap.com/ and search on what cryptocurrency that you want and then click the markets, there are the list of the exchanges that have the coins that you want.
Give Bittrex a try that is where most people started out.
No no no no, no bittrex. This site have many problems right now, the recent one is they changed the withdrawal minimum into 0 btc for basic account. So if you want use bittrex, you need to verification your id first to able to withdraw your coins,