It can't be converted but you can trade Steem for SBD (Steem Dollars) on the internal market at
https://steemit.com/market
or you could move Steem to Bittrex or another exchange, sell it for Bitcoin, then buy SBD.

SBD is quite inflated lately. It was designed to stay near $1 but it has done well n the marketplace the last several weeks hitting a high of around $15. The supply is being increased in order to bring Down the price to move it back toward $1 so I'm not sure if I would be buying SBD now.

Steem Power is locked up Steem. You can't do anything with it until you power it down to Steem. If you Power Down, you will get 1/13 of your Steem Power converted to Steem each week for 13 weeks, starting one week after you initiate it. But it's good to accumulate Steem Power because it determines your votes' value. You will make good curation rewards for your votes when you have a good amount of Steem Power and this provides easy fun residual income as you engage and vote for people's posts here on Steemit.

Welcome to Steemit; it's a great place to be!