Considering you used the brazil tag on your post, I'm assuming your fiat currency is BRL.
The cheaper and safer way I know to achieve this is the following:
Step 1: Open a mercadobitcoin.com.br account
This exchange is a bigger / older / most reliable on Brazil, I have an account there since 2014.
Step 2: Deposit your fiat currency:
Click on the deposit button, and you will be presented with the accounts where you can register and deposit the fiat currency.
It may take up to 3 days to confirm the deposit, but on all my experience, the money was released on the same day.
Step 3: Convert your Fiat Balance into LiteCoin
Litecoin is a faster and cheaper altcoin, which is supported by both MercadoBitcoin and
After changing to Litecoin on the dashboard, you may click the "Buy" and select how much, like o the following screenshot:
Step 4: Convert to Steem or SteemPower Using 
Open the page: blocktrades.us, there you can select the origin and destination crypto currencies.
I Think that covers most of it. :)
Feel free to contact me if you need help doing that, if you decided to do so.
RE: Help Me Invest My Fiat Currency Into Steem Power.