Your document class (which we don't know) may provide easy ways to customize sectioning headings. In my example, I use the standard book class and the titlesec package to change the properties of headings.
Notes: Instead of a fixed font size of 12pt, I've use the command \large which will produce a size of 12pt given the default standard size of book (10pt). Should you insist on a fixed size, I've added a \sizetwelvefixed command (to be used instead of \large). You also didn't specify if "normal" text should also by typeset in Times; in my example, Latin Modern is used for normal text and Times only for chapter headings.
\documentclass{book}
\usepackage{mathptmx}
\usepackage{lmodern}
\newcommand{\sizetwelvefixed}{\fontsize{12}{14.4}\selectfont}
\usepackage{titlesec}
% \titleformat{\chapter}[display]% OLD
% {\normalfont\huge\bfseries}{\chaptertitlename\ \thechapter}{20pt}{\Huge}% OLD
% \titlespacing*{\chapter}{0pt}{50pt}{40pt}% OLD
\titleformat{\chapter}[display]% NEW
{\fontfamily{ptm}\large\bfseries\centering}{\chaptertitlename\ \thechapter}{5pt}{\large}% NEW
\titlespacing*{\chapter}{0pt}{30pt}{20pt}% NEW
\begin{document}
\chapter{Introduction}
Some text.
\end{document}
You can check there for more sophisticated, but complex ways to change the appearance of headings.
– Johannes May 14 '11 at 10:01