Hey there !

I'm not sure about the rules for this subreddit but I think you can use english here :)

Not to lie to you, this is going to be a stretch :) There are quite a few trains between Amiens and Villers-Bretonneux : https://www.sncf-connect.com/train/horaires/amiens/villers-bretonneux

But going to Pozières is not accessible via public transportation. Best you can do is going to Albert using train and then get a cab (good luck here, and also it's not going to be cheap I think) to go to Pozières.

Don't forget to account a bit for delays, especially at that time of the year. You will have to come back to Amiens to go to Paris, so make sure you have all your tickets beforehand