makeinfo is a utility that converts a Texinfo file into an Info
file; texinfo-format-region and texinfo-format-buffer are
GNU Emacs functions that do the same.
See Overview of Texinfo, to learn how to write a Texinfo file.
See Creating an Info File, to learn how to create an Info file from a Texinfo file.
See Installing an Info File, to learn how to install an Info file after you have created one.