Normally, "the" function is the unit step function- u(x)= the largest integer less than or equal to x. As for finding its Fourier series, it is not periodic. I assume you are restricting it to [itex]-\pi[/itex] to [itex]\pi[/itex]. Now, just use the standard integral formulas for the Fourier coefficients. Because the step function is constant over each integer interval, you will just be integrating sine and cosine over 8 intervals.