The macro that generates the heading of the chapter (and what it looks like eventually) in the default book
and report
document classes is \@makechapterhead
(I've added some comments/descriptors):
\def\@makechapterhead#1{%
\vspace*{50\p@}% <----------------- Space from top of page to Chapter #
{\parindent \z@ \raggedright \normalfont
\ifnum \c@secnumdepth >\m@ne
\huge\bfseries \@chapapp\space \thechapter% <-- Chapter #
\par\nobreak
\vskip 20\p@% <-------------- Space between Chapter # and title
\fi
\interlinepenalty\@M
\Huge \bfseries #1\par\nobreak% <------------------ Chapter title
\vskip 40\p@% <------------------ Space between chapter title and first paragraph
}}
You can update these to your liking to modify the display. For example:

\documentclass{report}
\usepackage{lipsum}% http://ctan.org/pkg/lipsum
\makeatletter
\def\@makechapterhead#1{%
{\parindent \z@ \raggedright \normalfont
\ifnum \c@secnumdepth >\m@ne
\huge\bfseries \thechapter.\ % <-- Chapter # (without "Chapter")
\fi
\interlinepenalty\@M
#1\par\nobreak% <------------------ Chapter title
\vskip 40\p@% <------------------ Space between chapter title and first paragraph
}}
\makeatother
\begin{document}
\chapter{A chapter}\lipsum[1]
\end{document}
In the above example, the text block boundary was emphasized using showframe
.