Why does a photon have spin 1? Are we taking the photon spin to be one to describe electromagnetic force or there is any equation (is it relativistic Schrodinger or Dirac equation?) with a solution that tells us that its value is one?
 A: The question $\textit{why it has spin 1}$ is inappropriate. Particles, by definition, are embedded into irreducible representations of the Poincaré group, i.e., a field. Fields with distinct Lorentz representations have distinct phenomenology and so we must $\textbf{choose}$ the representation of the field in order to describe the correct phenomenology of the particle.
The photon is a particular case of this; it a boson with two degrees of freedom (two independent polarizations) which is its own antiparticle. In particular, the circular polarization of the photon is characteristic of massless spin 1 particles, since other spins like 0 or 2 have different polarization patterns. So, without going to deep into the theory, it is phenomenologically unavoidable to have a spin 1 photon.
A: Photons are spin-1 particles, they can be polarized in two different ways, circular (left und right), like electromagnetic waves. They obey the Maxwell equation $\partial_\mu F^{\mu \nu}=j^{\nu}$. The question why is not so easy to answer.
The photon is the U(1) gauge boson so it has to have integer spin. 
