In order to sell your Steem and convert it into fiat money, you'll need to first trade it into a cryptocurrency that can be traded into that currency.

If for example you use blocktrades you will be able to send your Steem from your account and exchange it for bitcoin.

The bitcoin can then be sold at an exchange of your choice for the dollars and then withdrawn in line with their procedures.

I mention blocktrades as they are the most straight forward exchange - you do not need an account.

Hope this helps!