IRAF is normally started up by issuing the cl command (you may need a login.cl startup file in your current or home directory). Converting an existing file in IRAF format into a FITS is very simple, as is illustrated in the following example:
% cl # startup IRAF
cl> dataio # go into the dataio area
da> wfits iraf_file fits_file # and convert it
da> logout # quit IRAF
%
The parameter bitpix may have to be set to 16 or 32, if you don't like it's default. The complementary IRAF program rfits converts a FITS file into IRAF format.