Florence is the best place in Italy about Renessance art, sculpture and architecture
You should visit Galerea Uficci to see paintings of Michelangelo, Karvagio and many many other popular artists, but be aware of HUGE QUEUES, its better to bye ticket in Internet to get in line for Reserved tickets.
The original of Michelangelo's David statue is placed in academy of fine arts of Florence ("The Accademia di Belle Arti di Firenze") - the queues are not big ( for me it was only 30 minutes).
Church Manta Maria del Fiore is very beautiful too.
Insider Tip:Don't bye anything in jewellery stores at Ponte Vecio, its extremely expensive and not art at all, they are just made for tourists - but its only my opinion. Historical part of city is very small, you can see everything in one day. One day more is need to see Galerea Uficci, because its has a really huge collection.