Hello, Welcome to Steemit.
If you want the full version answer you should check out the FAQs. But to answer your questions: Your STEEM Power and Steem Dollars can be 'traded' for STEEM. STEEM and Steem Dollars can be easily exchanged for Bitcoin (or other cryptocurrencies) using BlockTrades. Just go to your wallet click on your STEEM (or Steem Dollars) and select 'Sell'. Of course you will need a Bitcoin address for that (Since you are new to this, I suggest spending a couple of hours to get yourself up to speed on the basics of cryptocurrencies). After you have your bitcoins you can exchange them for a number of fiat currencies on one of the popular exchanges such as Coinbase or Poloniex (there are a lot so it's best to judge them yourself). That's the essence of it. There's a pile of other stuff just on the Steemit platform itself such as 'Powering Up/Down' and and the Internal Market, but this comment is getting too long for me. Just get yourself familiar with the cryptocurrencies's basics and read throw Steemit's FAQ section and that should clear out most questions.