How do I split a string?

You need to define a macro which has the separation character in the parameter text:

\def\testthreewords#1{\threewords#1\relax}
\def\threewords#1 #2 #3\relax{ First: (#1), Second: (#2), Third: (#3) }
\testthreewords{Now good enough}

If you want to be able to provide a macro as argument you need to expand it first. This can be either done once (only first macro is expanded once):

\def\testthreewords#1{\expandafter\threewords#1\relax}

or completely:

\def\testthreewords#1{%
    \begingroup
    \edef\@tempa{#1}%
    \expandafter\endgroup
    \expandafter\threewords\@tempa\relax
}

The \relax here is used as an end marker and must not occur in the argument, otherwise a different macro should be used, like \@nnil. The grouping is added to keep the temporary definitions local.

However this setup fails with an error if the two spaces are not included in the argument. To be on the safe side you should read every substring on its own and add the separation character to the end as a fail-safe. Then you test if the end was reached:

\def\testwords#1{%
    \begingroup
    \edef\@tempa{#1\space}%
    \expandafter\endgroup
    \expandafter\readwords\@tempa\relax
}
\def\readwords#1 #2\relax{%
      \doword{#1}%  #1 = substr, #2 = rest of string
      \begingroup
      \ifx\relax#2\relax  % is #2 empty?
         \def\next{\endgroup\endtestwords}% your own end-macro if required
      \else
         \def\next{\endgroup\readwords#2\relax}%
      \fi
      \next
}
\def\doword#1{(#1)}
\def\endtestwords{}


\testwords{Now good enough}% Gives `(Now)(good)(enough)`
\testwords{Now good}% Gives `(Now)(good)`

 

Another way : the words are stocked in macros \worda \wordb etc.

\documentclass[a4paper]{article}  

\newcount\nbofwords
\makeatletter  
\def\myutil@empty{}
\def\multiwords#1 #2\@nil{% 
 \def\NextArg{#2}%
 \advance\nbofwords by  1 %   
 \expandafter\edef\csname word\@alph\nbofwords\endcsname{#1}% 
 \ifx\myutil@empty\NextArg
     \let\next\@gobble
 \fi
 \next#2\@nil
}%    

\def\GetWords#1{%
   \let\next\multiwords 
   \nbofwords=0 %
   \expandafter\next#1 \@nil %
}% 
\makeatother

\begin{document}
 \def\MyTeXKnowledge{Not good  enough the end}
\GetWords{\MyTeXKnowledge}

There are \the\nbofwords\  words:  \worda; \wordb; \wordc;\wordd;\worde.

\end{document} 

Now \MyTeXKnowledgeis accepted.