I'm confused about how to calculate noise for an SNR?
Where does noise come from?


Any electrical resistance above absolute zero produces thermal noise.


  how can I calculate it?


It can be modeled as a small AC voltage source in series with the resistor. The thermal voltage RMS value is: -



Where k is Boltzmann's constant, T is temperature in kelvin, R is the resistance and B is the bandwidth. You could also use a calculator like this one.