One of your first stops should be Office de Tourisme du Pays de Mayenne. Here, you'll be able to orient yourself and get lots of useful information before you set out exploring.
This tourist information center is just one stop you'll want to make in Mayenne. Lassay les Chateaux is conveniently near Office de Tourisme du Pays de Mayenne and will be a great addition to your travels through the area.