I want all my headings to have these characteristics:

1. 12pt

2. Center aligned

3. Bold

4. Times new roman font

But when I write \chapter{Introduction}, I can only manipulate the name of the chapter, not the text saying Chapter 1. My final result should be (centered):

Chapter 1

Introduction

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Give some more information, eg which documentclass are you using? – Herbert May 14 '11 at 9:28
This seems to be kind of a special case of the question asked here: tex.stackexchange.com/questions/11444/… You can check there for more sophisticated, but complex ways to change the appearance of headings. – Johannes May 14 '11 at 10:01

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}

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Thanks a lot, lockstep! I was going crazy with this thing :) – Sérgio May 14 '11 at 10:35
Thank you for this code.But when I put it in my file the title "Table of content" doesn't change. Could you help me, please? – filokalos May 30 '11 at 18:36
@filokalos: You don't mean the "Contents" heading but the chapter entries inside the ToC, right? – lockstep May 30 '11 at 18:40
Frankly, I don't know. :) Sorry. Maybe it's "Contents" heading. It appears when I give the command \tableofcontents. – filokalos May 30 '11 at 18:53
@filokalos: Ask a new question (minimal example included), and I'll have a look at it. – lockstep May 30 '11 at 18:55