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.
LaTeX Content, write the code for your template; the code could be something like this
\tableofcontents % for a table of contents
@ is for TexMaker dot.
To use your template, open new file go to
User > User Tags, and choose the user tag you want.