You can trade STEEM for SBD, and SBD for STEEM on the internal "Currency Market". There is a link for that in the main menu.

If you want to buy STEEM or SBD using 'real money', you first have to buy Bitcoin, Litecoin, or Ethereum first. I'm not sure where you live, but coinbase.com is the main website where people buy it if they live in the US. (Otherwise you can just google where to buy BTC in your country if you live somewhere else.)

Once you have one of those three coins, you can use them to purchase STEEM or SBD using https://blocktrades.us/.

Hope that helps! Let me know if you have any other questions :)