% Dans le préambule
% -----------------
\usepackage{tikz}
% -----------------

\begin{tikzpicture}[grow'=right,level distance=2cm, sibling distance=0.8cm,->]

\node {A}
  child { node {B}}
  child { node {C}}
  child { node {D}
            child { node {E}}
            child { node {F}}
        };
\end{tikzpicture}