There are two options for you:
Take train from Fuzhou and get off at Yongding. Then take taxi to Yongding Bus Station. There are buses to Gaotou passing through Yongding Tulou.
Take bus at Fuzhou Bus Station to Longyan. Longyan Bus Station has special tourist bus to Yongding Tulou.
I guess the second one is better, seems to be easier for you. It takes 5 hours from Fuzhou to Longyan. I think you should stay one night in Yongding or Longyan. If you are not confident in making this trip by your own, find a reliable local tour agent with good price. That's the best for visitors who never come before.