See this section of Marriott Rewards for a start to converting points to miles. Hope your airline is a partner and good luck!
PS--takes a little while (4-6 weeks) unless you're willing to incur (in some cases) an expedite fee.
Having recently done this on Delta, let me share with you that the flights and points required change HOURLY!
When I started looking for a bus class to CDG the end of April Delta was 200-250,000 Delta points each. Just by checking daily the points required dropped to 150,000 each (AND I BOOKED THEN).
Key, have your passport ready and stay on top of the changes. You will want to make sure you book your flights exactly as your name appears on the passport. One last thing, I am a big AMEX customer and they helped me with Delta as the flight and points required literally changed in the middle of booking the flight.
Good luck, and also be aware that some airlines are requiring less points on your trip than Delta.