Ciao Jashley, welcome to our foum!
It's super easy to get to the city center from the Villa Costanza stop that Flixbus is now using for all buses arriving in Florence.
Just use the tram! The T1 line starts there and it takes you directly into the center to the SMN train station (stop Alamanni or Valfonda).
If you're still far from where you need to be, then you can go to the taxi stand in front of the station and catch a taxi from there. It will cut the cost of the taxi fare, that's for sure! The tram costs 1.70 euro for 90 minutes, the same ticket can then be used to get on a bus, if you see there is a bus near where you need to go you can do that rather than the taxi.
Let me know if you have any other questions!