This is how can you read the contents of the file into variables.
data_file = websave('phi_theta.txt','https://www.mathworks.com/matlabcentral/answers/uploaded_files/1315990/phi=0%20and%20theta=10.txt')
fid = fopen('phi_theta.txt')
datacell = textscan(fid, '%f %f','CommentStyle','#');
var1 = datacell{1};
var2 = datacell{2};
plot(var1, var2)
Regarding the fft, please have a look at the Matlab documentation:
https://www.mathworks.com/help/matlab/ref/fft.html