The Derogatory Marks should state who/where you owe Money too...if not, then like Jen says appeal to get them off your report. And why exactly are you trying to Fix your Credit?
You an apply for Medicaid if you like...if not, try getting some Over the Counter Alternatives
As far as investing, start by getting a Stable Bank ACCount at a Major Bank or Credit Union...then, if you like you an CheCk out plaCes like you mentioned, Robinhood, also Stash and if youre looking for something 'Safe', try a High Yield Savings or Mutual ACCount