How to typeset in initex?

There are a lot of basic values that are initialised by the plain format so are not set here. In particular, you need a non-zero \hsize and some \parfillskip stretch:

\hsize=10pt\parfillskip=0pt plus 1 fil\font\f=cmr10\f a\end

Just to show a slightly different approach, if you don't set the hsize you can avoid an overfull box by not starting a paragraph

tex -ini '\catcode`{1\catcode`}2\font\f=cmr10\f\hbox{a}\end'