Thank you for visiting.
I have actually never bought steem, but as I understand it can be treated as any other crypto-coin.
If for instance you have bitcoin, you can use it to buy steem.
Via blocktrades, after your fill in the amount of crypto-currency (such as bitcoin), it will let you know how much steem you are buying.
Afterwards, you put your steemit username and then click on "get a new receive address"
Then you receive a memo which you need to include when sending bitcoin over.
Then blocktrades will do the transaction for you and deliver it to your steemit account.
You can visit blocktrades here or from your wallet > Buy or Sell Steem