If you want to trade your steem, blocktrades.us is probably the easiest way (we support a bunch of different trading pairs for steem). If you want to trade on a regular exchange though, OpenLedger or Bittrex both support trading to BTC. I haven't used Bittrex, but to send it to OpenLedger, you send to "openledger" with a memo of "open.steem:xxx" where xxx is your OpenLedger account name .