If you are getting interference fringes in transmission, you can use Swanapole's method to find out thickness and optical constants including real and imaginary parts of refractive index, absorption coefficient as a function of wavelength which in turn can give you optical band gap from Tauc plot.In this method you need not to know order of fringe.As far as your question is concerned first observed fringe will correspond to first order if you are taking spectrum with increasing wavelength. For Swanapole's method, you can go from 200 nm to 2200 nm to see goog number of fringes.

I too suggest to use swanapole's method which is easy to use if you have good number of fringes.

i will suggest to use the swanapeols method, envelope method to find out the optical constants