Florence could be visited in so many ways... it all depends how much time you can spend in town, if you are interested in any particular museums, if you are a guide book person or not...
However, having a private Guide gives you a completely different flavour to your trip, add a new dimension to things, and really makes you understand and appreciate the city.
One of the way, and one of the most popular tour that I do, is the Classic City Walking Tour; we will walk around the city center, skipping the big and famous streets that you can easily explore by yourself, and entering into the deep heart of Florence.
Trying to keep a chronologically way, I'll explain you the Roman origin of Florence, how the city was considered the Tuscan Manhattan during the Middle Age, and finally talking about the Renaissance, visiting the tipical nobleman house, seeing how incredibly Brunelleschi builded the Cupola, surely mentionning the Medici and their relationship with Florence.
Tour lasts 3 hours normally, but it can be surely longer, including some entrances into churches/museums.