Minimum (GNU Texinfo 6.7)

2.3 What a Texinfo File Must Have

By convention, the name of a Texinfo file ends with one of the extensions .texi, .texinfo, .txi, or .tex.

In order to be made into a printed manual and other output formats, a Texinfo file must begin with lines like this:

\input texinfo
@settitle name-of-manual

The contents of the file follow this beginning, and then you must end the Texinfo source with a line like this:

@bye

Here’s an explanation:

The ‘\input texinfo’ line tells TeX to use the texinfo.tex file, which tells TeX how to translate the Texinfo @-commands into TeX typesetting commands. (Note the use of the backslash, ‘\’; this is correct for TeX.)
The @settitle line specifies a title for the page headers (or footers) of the printed manual, and the default title and document description for the ‘<head>’ in HTML. Strictly speaking, @settitle is optional—if you don’t mind your document being titled ‘Untitled’.
The @bye line at the end of the file on a line of its own tells the formatters that the file is ended and to stop formatting. If you leave this out, you’ll be dumped at TeX’s prompt at the end of the run.

Furthermore, you will usually provide a Texinfo file with a title page, indices, and the like, all of which are explained in this manual. But the minimum, which can be useful for short documents, is just the two lines at the beginning and the one line at the end.