Aisne-Marne American cemetery

Trains going in that direction leave from Paris Est so you would have to take the metro or other means from Nord to Est. Trains go to Nanteuil Saacy or to Chateau-Thierry, from either of these you could take a taxi or check to see if there's a bus to the cemetery. Not particularly easy without a car.

EDIT: this info is from Rome2rio

Directions from the website:
https://www.abmc.gov/cemeteries-memorials/europe/aisne-marne-american-cemetery#directions-anchor

Slightly off topic , but you would have more flexibility with a car , driving in the area is easy . Nearby is another cemetery at Oise - Aisne ( about twelve miles from Aisne - Marne ) , Joyce Kilmer is buried there . If your interest includes the Second war , it was also ( until 1987 ) the burial site of Eddie Slovik . His story is notable and saddening , you can read this book to learn about it - https://www.amazon.com/Execution-Private-Slovik-William-Bradford-ebook/dp/B00KYL66IA/ref=sr_1_1?ie=UTF8&qid=1542044932&sr=8-1&keywords=execution+of+private+slovik A brief overview here - https://en.wikipedia.org/wiki/Eddie_Slovik

If you do go by train, Gare de l'Est is only a short walk from the Gare du Nord. You cannot usefully go between the two by metro.

thanks, all these responses are very helpful.

that taxi from the train to the cemetery....i wonder how much it is supposed to cost?

It is about 7 miles. I'll hazard a guess of 20 EUR one way. It is 16.90 EUR one way for the train from Paris Est, or 12.70 EUR if you are 60 or older.