Zhangzhou - Xiamen

Hi!
i want to go from Zhangzhou to Xiamen down town, i read its convienent to go by bus.
can you help me, where is the bus station to go? because usually 1 city they have a lot of bus station.
an which bus station in Xiamen i should take, if i want to go to downtown?
we problably will stay near the zhongshan area.
how much does it cost, and how much time does it take from Zhangzhou to Xiamen city center.
and is it easy to buy the ticket? can we just go show and buy the ticket or better to buy the bus ticket in advance. if we have to buy in advance, please inform me where to buy.

thank you
Fenny