If you have your STEEM on an exchange transfer it to your account here on Steemit. Then in the top right click on the 3 horizontal lines and go to market. From here you can see a live feed of what you will be getting for your STEEM/SD. Under sell STEEM you can see what the current highest bid is. Currently it is 2.249948 so for every 1 STEEM you sell you'd get just shy of $2.25. Once you have SD in your account you don't have to do anything to gain interest on it. Hope this helps you.