OK so first Welcome to Steemit !
Second @tobixen is right you put 1 steem and you will gain 1 steem power to your account.
Steem and steem dollars can be traded any time while steem power has a specific rule and making it NOT tradeble at any time but when the time is right you can power down and gain some $ in your wallet.
I sugest to hold your Steem Power and not use it because, SP influences your voting power and thus posting power too, that means when you post something it will be more visible to more users here on Steemit, and last but not least steem value is in a constant flux and by holding sp in your wallet you can receive some % of steem...just like that...holding Steem Power.
I hope that my little post helped you in some way , I followed you and I wish you happy posting !