There are three main options for buying Steem:
1) Via blocktrades, an "instant exchange" very simply converts your BTC or other altcoin to Steem or Steem Power . View the tutorial for Blocktrades (recommended!)
2) Via Bittrex, a normal "cryptocurrency exchange," buy and sell Steem on their market and withdraw to your steemit account. View the tutorial for Bittrex
3) Via OpenLedger, a new type of "decentralized exchange," buy and sell Steem on their internal market and withdraw to your Steemit account. View the tutorial for OpenLedger
Hope that helps! Feel free to ask about how to Buy and Transfer Steem below. :)
