As I was sleeping that was actually a slow reply from me😁

Steem Power doesn't degrade no, it will only rise when you either earn curation rewards or Power Up your STEEM.

SBDs are best traded on the 'Market' (accessible from the dropdown menu next to SBD), if you want to by STEEM to power up.

And no, you are not delegating to anyone, Steemit are delegating to you (the number in brackets) to start you off.

Hope that helps!