I have a daily data from a few years and I need to model it somehow. As I see seasonal factor I thought about truncated fourier series:
s(t)=a+bt+a_0+ \sum_{i=1}^{I_1}a_i sin (2 i \pi (t- f_i)/365)+\sum_{j=1}^{J_1}b_j cos (2 i \pi (t- g_i)/365)

But i don't know how to start and what program use to fit data to this model and find coefficient?

Thank you in advance for any help