Here's anexample for getting started with the GA. My guess is that you're trying to find the optimal parameters for your FLC that result in the system's response being as close as possible to a target response. If that's the case, you will need to write a fitness function that computes the difference between the response of your system and the desired response.
The input to the fitness function will be the vector of the different parameters for your FLC. The fitness function will then simulate the system with these parameters, and calculate how close the simulation was to the desired response. There's an example along those lines in thisvideo, where the goal is to find the set of parameters that minimizes the difference between the simulated response and the desired response.