You need to transfer your steem from your wallet(using the transfer button). You'll need to create an account at an exchange such as bittrex.com or poloniex.com to transfer it to.
From there you can trade on the exchange for bitcoins. Finally transfer the bitcoins to coinbase.com, and turn them into USD.
It sounds like a big pain, but once you have everything setup, it really isn't too bad. Hopefully as this thing takes off, there will be better tools that appear!