If you are new in the crypto currency world, the first thing you need to do in order to buy your first bitcoin is to create an account at some of these exchanges: http://usbitcoinservices.com/affiliates/
or if you don't know which one to select, just go to circle.com and open an account there. Here you need to attach your fiat (USA dollar) bank account, so the funds are pull from there to your circle account.
Once there you buy bitcoins.
The second step will be to send the bitcoins you bought there to an exchange like Poloniex, or Bittrex
Once you are the exchange, you need to buy Steem using your bitcoins you just bought.