The Gare du Mans (Le Mans main station) lies approximately 1km south of the city centre to the east of the Sarthe river. It is housed in a modern, rather uninspiring building, however it does have a comprehensive range of services and amenities including luggage trolleys, a creche, a photo machine, good disabled access, etc..
You'll probably want to take the bus or tram into the town centre via the north exit, as it is otherwise a decent walk away. Setram is the company that runs the public transport network, and there are four bus lines operating from the station. Line 5 takes you right into the city centre.