Maybe a link to the bike you are looking at would help. The Craigslist in my area seems to have good used bikes from $100-$200 pretty regularly, but you need to be ready to act fast. You can get an entry level hybrid or Cruiser bike from a bike store for about $250 though. If you don't know anything at all, that may be a better option, because you know the bike will be ready to ride when you pick it up.
If you don't know how to ride, you may want to look up The Historian, he is a regular poster in the Cydes forum and learned to ride as an adult less than 2 years ago and has had some great adventures....anyway he may have some advice for you about learning to ride.