The battlefields of Western Front cover a large area from the coast in Belgium to the Swiss border, more than 700km ! There are many different WW1 battlefield along the Western Front, the Somme Battlefields and the Arras-Vimy Battlefields are easily accessible coming by train from PARIS for the day and the trains run very often.
The city of AMIENS is a good base to visit the battlefields of the Somme. It's 120km north of PARIS and 100km from LILLE. If you are coming by car the traffic can be heavy on the morning at AMIENS and parking a problem too. Coming by train is easier and faster.
Coming from PARIS catch the train at Paris-Nord station ("Gare du Nord") to AMIENS, it takes 1h10. No seat reservation or booking, just buy an open ticket on the day. The trains from PARIS to AMIENS run every hour during the week.