You can buy Steem on two exchanges currently: Bitshares (https://bitshares.openledger.info) or Bittrex. On Bitshares you need to buy OPEN.STEEM, which can be transferred to your Steem account. However, the easiest and fastest way currently is probably to buy Steem Power directly from Blocktrades (they also sell Steem). This means they transfer the Steem directly your account in the vesting form, which is what gives you voting power and protects you from inflation.

https://steemit.com/blocktrades/@blocktrades/blocktrades-now-offering-steem-power-for-cryptocurrency
https://steemit.com/steem/@blocktrades/-blocktrades-adds-support-for-directly-buyingselling-steem