Built by Michelangelo in 1524. This is the final resting place of some of the most famous Medici's including Pope Clement the VII
St Lorenzo's Church contains the The Medici Chapels ('Capelle Medicee'), where some members of this powerful family are buried.
Hours: Tue-Sat 8:15am-5:00pm
I couldn't visit Palazzo Medici-Riccardi when I was in Florence because it was closed.
I believe there is more than one entrance.
Go around the building and try the one on via Camillo Cavour.