You have to transfer it to an Exchange website, some people want to buy bitcoin for steem, some people want to buy steem for bitcoin, depending on how they view the market and their own finances. So you basically do a swap. You sell them Steem, they sell you Bitcoin. To do this you have to send your Steem, or Steem Dollars to an exchange, either Poloniex or Bittrex are good choices. On Poloniex go to the deposits & withdrawals section and next to Steem (or SBD) click deposit, there you will find a memo and an address, the address is Poloniex. Go back to Steemit and in your wallet enter the memo and the address (poloniex), and an amount (i recomend a small amount to start with) and click send. It will take a while to come through, maybe 5 minutes (bitcoin could be much longer) but once its there you go to the exchange section and click Steem in the column at the top right. Then below the chart, on the left half way down, you enter the price in BTC you wish to sell for (currently 0.0058) and the amount of steem you wish to sell. It will then calculate how many bitcoin you will recieve, place the order. You can also set a higher price, but it will only go through if the market goes up (it costs slightly less in transaction fees this way)
Hope that helps, its not something a 5 year old could do really. :)