I just started whitewater nearly a year ago to the day. You've gotten some solid advice so far - don't buy new. Buy AS USED AS POSSIBLE. I had my first boat for 2 months before I traded up. I went from a very used Wavesport Y (6 months), to a slightly used Stomper (4 months), to new Burn 2.0 (4 months and counting - got the Burn 1/2 price in an online auction). A boat like a Stomper or a Jackson Hero will be extremely forgiving and allow you to ease into whitewater. My friend got a Mamba 8.1 and wasn't ready for how 'tippy' it was.
Two cool things about buying a used boat - 1.the guy who owned it before you may become your new paddling buddy - 2. used creekers/riverunners really don't lose much value, so you can buy a used one for $400 and probably sell it in 4 months for $350.