You can only buy ETH, BTC, and LTC on coinbase
So what you do is deposit your USD from your bank account to coinbase, then buy ETH on coinbase, then transfer the ETH to an exchange like Bittrex and exchange the ETH for whatever shitcoins suit your fancy
The downer about this is that is will probably take a week or more to get your ETH from coinbase, but the price is locked in as soon as you hit buy
Also remember to track your buys and sells and know your cost basis on each trade for tax purposes