buy Steem from cryptocurrency exchanges (Binance, Bittrex, Poloniex etc.) and send it to your Steem wallet here. Then simply power up. Steem/Eth pair is available on Binance. However, you can sell Eth for Bitcoin and then buy Steem from Steem/Btc market as well.You can use Blocktrades as @timcliff mentioned. If you are interested in alternative, you can

Steemit account wallet is your Steem wallet and @chidimicheal is your wallet address (Steem username).