Can you tell what you have beside the Mega 2560 board ? Or show a photo ? Do you have a working sketch ? Can you show it ?
What kind of 8x8 matrix is it ?
I agree with the others, it is better to start with something that you can do. But there are many examples for 8x8 leds. Perhaps with an example you can show your own design on the leds.
There are examples of beautiful creations. This is an example with a 6x10 NeoPixel matrix: https://wokwi.com/arduino/libraries/FastLED/Pacifica. Click on the start button in the middle-upper of the screen.