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.