If you have Bitcoin or Ethereum, or other digital currencies, you can do it from your wallet by clicking the box that says BUY STEEM OR STEEM POWER.

Another way is to get an account on Bittrex. I'd actually suggest doing it this way as Bittrex is a US company and fairly trustworthy. If I understand correctly, you can set up a bank wire with them.

Another method would be to sign up with Coinbase. They're also US based and trustworthy (your USD in it are FDIC insured) and you can buy Bitcoin or Ethereum there and then use Blocktrades to trade that for Steem.