Rola,
If you click
HERE, you will download a pdf file which is the route guide to the Adirondack train which travels from Montreal to New York City. On page 4, the same brochure is available in French. There is actually a stop in St. Lambert if you are closer to that city.
The first passenger stop in the US is Rouses Point, New York. It is about an hour and a half from Montreal. One way cost is only about $16 (subject to change based on demand). However, this is a very small town, and there may not be any shopping available. With only one train per day, you can't get off the train and wait for the next one to take you deeper into New York. It is good to plan ahead.
Plattsburgh, about 3 hours from Montreal and the 2nd stop in the US, has a lot of shopping geared towards Canadian tourists seeking to spend their strong Canadian dollar. One such shopping centre is
The Champlain Centre. It doesn't appear to be a far taxi ride from the train station. You should arrive around 12:30 PM. The return to Montreal leaves the station at 3 PM. This doesn't leave much time for shopping after taking into consideration delays and transportation in town, so I would recommend staying the night. There should be plenty of lodging available.
Of course, if you choose to spend the night, then that opens up the whole state of New York. I recommend looking at the guide that I linked to at the top of this message and researching the various cities along the route to make the best choice for your needs.
Good luck and I hope you enjoy your experience.