check your transmisson fluid level, and check trans sump seal for leaks, if leaking seal need to change it, if low fluid level, top up, start engine, keeping the brakes on, go though gears to settle fluid, if this dont work you will need to go to an automatic transmisson garage, and this aint cheap. did you know about gearbox when you bought it cos you can bet the seller did. Good Luck.
hope it helps
July 2010