Getting to Montreuil-sur-Mer
Montreuil-sur-Mer is about three hours from Paris by train. Tickets are available in advance through SNCF, the company that controls the French rail system, or in person at a ticket booth. Prices vary by schedule, averaging around 50 euros for a round-trip ticket from Paris. Since Montreuil-sur-Mer is on a regional line, you'll probably end up taking the TGV to a hub station--probably Arras or occasionally Béthune--and transferring to a local train. If your transfer point is Arras, you'll pass through Saint-Pol and Hesdin, Valjean's stopping points on his way to Champmathieu's trial, and you'll discover that the journey that took him eighteen hours in 1823 takes a little over an hour today due to the wonders of modern technology.
Montreuil is divided into the upper city and the lower city by a steep hill and an impressive rampart that surrounds the upper city. The train station is in the lower city and the town is too small for a bus line, so leave that fifty-pound wheeled suitcase behind unless you want it to wrench your arm off as you drag it up a very steep cobblestone street. Maps are available in the tourist office near the citadel, but Montreuil is a very small place and it's impossible to get lost--if you don't know where you are in the upper town, wander until you reach the ramparts and follow them back to the citadel.
If you want to stay overnight, there are combination restaurant-hotels everywhere. You might also want to try the Hotel de France, the oldest hotel in Montreuil and the place where Victor Hugo allegedly stayed when he visted in 1837--ask for room 12b.