I buy bitcoin on Coinbase so that I can use it in other places if need be; It lets you purchase BTC, ETH and LTC with USD which as @dutch pointed out is difficult in most places. Then you can use something like ShapeShift.io to send some "ETH" to your Steem wallet address; which you'll find in your wallet -- You'll find your steem wallet and can easily 'shape shift' money from your Coinbase wallet, to your Steem Wallet; you can also buy Steem from an Exchange as well; I use Bittrex but there are a few other trusted sources that people on here like to use as well.