Could anyone give me a god site to book a train - overnight - from Amsterdam to Poland.
Thank you Ronda

You can try the Dutch Railways international site: http://www.nshispeed.nl/
For a random date in December I found the price and was offered to buy the ticket. So it seems your best choice at the moment, as the Polish intercity operator only offers international tickets to/from Berlin.
Mind you, the NS Hispeed website is often a bit slow ;).
You can also try flying to Poland. Cheap airlines leave from Eindhoven (Netherlands), around 2 hours from Amsterdam Central station (1:20 by train, and 20 minutes by bus). Cheap airlines include Ryanair, Wizzair, and Easyjet (don't know if Easyjet cover that route). You can search momondo.com or skyscanner.net for cheap flight tickets for regular airlines flying from Amsterdam Airport (KLM or LOT).
As far as I know the train is more expensive than the flight - the train is something like 130 euro one way, at least twice as much as the flight (at least in December).

You can also book them in the web of the German Railways. You can consult here the details.
the train is something like 130 euro one way
It can be much more cheaper than that. Take a look at the link I provided above.
You can also try Wasteels. They have offices throughout Europe and also a web site.
Good luck.

Where do you want to arrive in Poland? If it's Warsaw, you can take a direct night train from Amsterdam to Warsaw, departing at 19:01 and arriving at 10:55. If you book ASAP (up to 92 days allowed) on the German Rail site, you can get a discount Saving-Night fare for a bunk in a 6- or 4-person couchette or a bed in a 3-, 2-, or 1-person sleeper. All couchettes are the same size. The only difference is in how many bunks are pulled down. You'll have a lot more room for both people and luggage in a 4-person chouchette.
That trip passes entirely through Germany so use www.bahn.de. There's a direct overnight train. I don't know when your trip is, but right now you can get a seat for 30 euros, a chouchette for 50 euros -- extremely cheap for a 16 hour ride. The regular price is 134 euros, which is what you'll pay if you don't book a few weeks in advance.