Before worrying about coding it in 'C', do you understand the basic principle involved?

You just need to be switching a pin on and off - the more on than off, the brighter it will be.

You could do that with software loops, or with a standard timer, or a PCA, or dedicated PWM controller.

But you need to understand first what you need to do - the detail of how to code that in any particular language should just follow.

What is your particular difficulty?