Go to User > User Tags > Edit User Tags
like in the following picture:

Choose a menu item, and give it the name in Menu Item
(on the right), with which you want to identify this template.

In LaTeX Content
, write the code for your template; the code could be something like this
\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{lmodern}
\author{@}
\title{@}
\begin{document}
\maketitle
\tableofcontents % for a table of contents
\section{@}
\end{document}
@
is for TexMaker dot.
To use your template, open new file go to User > User Tags
, and choose the user tag you want.
New by copying existing file
and theWizard
menu option(s). But I wouldn't say those are 'templates'