How to set document font to times new roman by command

Let's construct what you're after:

1. 12 point text font

   For this you can pass a 12pt option to the document class. For example, use

   \documentclass[12pt]{book}
   

2. Times New Roman font

   While there is no actual Times New Roman font in native LaTeX, the closest you'll get is by adding the mathptmx package

   \usepackage{mathptmx}
   

   or the newtx bundle

   \usepackage{newtxtext,newtxmath}
   

   to your document preamble. To check the actual font is included when doing this, see How do I find out what fonts are used in a document/picture?.

3. 14 point chapter heading

   Under the 12pt document class option, the closest to 14pt is provided by the \large switch (see What point (pt) font size are \Large etc.?) - it'll actually be 14.4pt, but that would be what the font has to offer. However, the current \chapter heading is set (by default) in a combination of \huge and \Huge (the former for the chapter heading - Chapter X, and the latter for the chapter title A chapter). So, we can patch the appropriate macro(s) (\@makechapterhead and \@makechaptershead) and substitute \large for \huge (with the aid of etoolbox):

   \usepackage{etoolbox}
   
   \makeatletter
   % \patchcmd{<cmd>}{<search>}{<replace>}{<success>}{<failure>}
   \patchcmd{\@makechapterhead}{\huge}{\large}{}{}% for \chapter
   \patchcmd{\@makechapterhead}{\Huge}{\large}{}{}% for \chapter
   \patchcmd{\@makeschapterhead}{\Huge}{\large}{}{}% for \chapter*
   \makeatother
   

Here's a MWE that contains the above suggestions:

\documentclass[12pt]{book}

\usepackage{lipsum,mathptmx,etoolbox} % Or swap mathptmx with newtxtext,newtxmath

\makeatletter
% \patchcmd{<cmd>}{<search>}{<replace>}{<success>}{<failure>}
\patchcmd{\@makechapterhead}{\huge}{\large}{}{}% for \chapter
\patchcmd{\@makechapterhead}{\Huge}{\large}{}{}% for \chapter
\patchcmd{\@makeschapterhead}{\Huge}{\large}{}{}% for \chapter*
\makeatother

\begin{document}

\chapter{A chapter}
\lipsum[1]

\section{A section}
\lipsum[2]

\end{document}

Note how the \section title is now larger (set using \normalfont\Large\bfseries) than the \chapter title - a definite problem. However, you requested Chapter Heading 14 only.

In order to have 12pt for your document - this works well

\documentclass[a4paper,12pt]{report}

Use this at the start of the page. There is no predefined times new roman so you can use this instead -

\usepackage{Times} or \usepackage{mathptmx}

I was informed the times one is outdated and after trying the other one there were no visible changes so you can use any of them if you like. Hope this helps