There are literally hundreds of churches in Florence, but if you are going to just visit one, make it Santa Croce situated of course in Santa Croce Square.
The website linked above will tell you so much more, but generally it is open on weekdays from 09:30 until 17:30 and holidays and Sundays between 14:00 and 17:30. In both cases, the last admission is timed for 17:00.
Admission charges are around €6 but as you would expect, there are oncessions and party rates , family rates etc.
You can take a guided tour with headphones at reasonable rates and take about an hour.
As someone said, if you go to Florence and do not go to Santa Croce Church, then you have not been to Florence,