If you want to power up, you can invest in STEEM then power it up. If you invest in SBD it must first be changed into STEEM to power up.
You could set your rewards when you post to 50/50 that way you will get STEEM Power and SBD or you could set it for 100% STEEM Power in which case every reward you get will be powered up and increase your share of the vote pool, I'm not sure if it will increase your reputation too, I think that is only by posting and commenting.
If you want to take out rewards from Steemit, an easy way is blocktrades.us there's no need to sign up. Before you do that though, set up a wallet for the token you want to trade into (I use Exodus, it's easy to understand and has a few tokens, BTC, ETH, LTC, EOS, etc).
You also have the option of exchanges such as Bittrex etc if you want to use them, there have been problems with some of those though but if you want to trade then they are your only option, I just buy using blocktrades.us and hold in my exodus wallet. It's been scary lately.
I haven't tried to take real fiat money, that seems a lot more difficult and besides I want my mix of cyber currencies to grow over time so I'm HODL what I get.
I see you are talking about investing $ so I guess you have that part figured out.
Good luck to you.