Ciao Carmen! I know, there are way too many places to choose from in Florence, it can get overwhelming. If you already know you'd like a B&B, then you've narrowed the search a bit.
Take a look at this page: http://www.florenceaccommodation.com...nd-breakfasts/
and see what you like. On this page, I'd highly recommend Casa dei Tintori as a special place where you get to meet the owners and have a very cozy atmosphere, while the Residenza Casanuova also offers a stay at a small, family-managed hotel but in a more residential quarter, quieter and with the local market where residents shop at nearby. All of the others are very nice, just depends on what you're looking for.... all are in the center so very central to all of the sights. If you have any questions on any of them or are looking for something in particular, let me know and I'll tell you which ones you should look at.