I haven't done any online projects recently, but for some years I used to work pretty much exclusively on projects from rentacoder.com (now vworker.com).
The way I got into it was by starting bidding low on small jobs, getting good feedback, and progressively moving onto larger jobs. You'll find that the people willing to pay a decent amount on these websites also want experience and good reviews.
However you approach it, don't be discouraged when you don't win projects. It takes a while to get into the game. And regardless of how well you do, remember that you'd still make more money by working for locals (which is why I quit). Unless you enjoy it, theres probably better ways of making money.