I have not been down that road, hopefully someone who sees this will respond and we can both learn together. I believe you set up with an exchange to do that and here is the link in the faq's that give the info they have, but with over 200 different exchanges I'm sure there are many firms to do this.
https://steemit.com/faq.html#How_can_I_withdraw_my_STEEM_or_SBD_coins
How can I withdraw my STEEM or SBD coins?
STEEM and SBD tokens are readily tradable to bitcoin, which is readily tradable to the local currency of your choice. There is a link to "Sell" your STEEM and SBD tokens in your wallet, which uses the BlockTrades interface.
There are several guides that have been posted by users in the community for using various external exchanges to withdraw STEEM and SBD tokens. Please read the disclaimer before using any of these guides to withdraw your coins. The users, guides, and exchanges listed in the guides are not endorsed by Steemit, Inc. Use the guides below at your own risk.
It is recommended that you withdraw a small amount first, to verify it works before withdrawing a larger amount.
Sell Steem Dollars via Poloniex
https://steemit.com/steemit/@ash/steemit-how-to-sell-steem-dollars-via-poloniex-newbie-friendly
Withdraw Steem Dollars to a Bitcoin address
https://steemit.com/steem-help/@piedpiper/how-to-withdraw-your-steem-dollars-in-less-that-a-minute
Convert Steem Dollars to a country’s currency and withdraw to a bank account
https://steemit.com/tutorial/@beanz/how-to-get-my-usdteemit-money-into-my-bank-account
Convert STEEM to many other cryptocurrencies via ShapeShift