Ok.... You need coinbase to buy eth or Bitcoin. Then find an exchange like bittrex or many others. The exchange will allow you to buy other coins with your bitcoin, they don't take cash so you buy Bitcoin at coinbase first...you'll learn to transfer, also you can then transfer to a private wallet for your coin so you can securely hold it off the exchanges. If you want to sell say your Steem you send it to the exchange and trade back into Bitcoin and then send it to your coinbase which is where you can trade Bitcoin for cash ..... So think of Bitcoin and ethereum as the reserve cryptocurrencies. Coinbase is to buy Bitcoin and ethereum with cash and also to cash out..... But you need an exchange like bittrex to dabble in the other coins.