Florence airport to the main train station (Firenze Santa Maria Novella)

I'd recommend taking the bus...it's a lot cheaper, and so easy. It's about 3Euro if I remember correctly.

Tickets are for sale inside the airport or you can purchase from the bus driver...and you get the bus just outside to the right. Look for the blue metal posts. The bus will let you off just behind the train station in Florence central.

Will it be safe to take the bus at night at around 11:00pm?

I agree with Ellen, take the bus. It's only a 6-7 mile express from the airport to the bus station. A cab will be 20Euro-25Euro and isn't bad if you go directly to your hotel as we did. Note that if you take a cab, there is a supplemental fare fee for luggage and another for changing cities should that occur(i.e. Florence to Fiesole).