How can someone power up with US Dollars?

I currently have no BTC or Altcoin currency and am interested in powering up?

I am thinking I might have to buy some BTC somewhere with US Dollars and turn aroung and use that BTC to power up my steemit account?

I went to bittrex.com and being new there am not certain how I'd go about making this work.

Can anyone give some help to newbs in this area? I am certain I am not the only one that would like to know this.