LaTex makes it easy to divide a document into (chapters and) sections, using

\chapter \subsection \paragraph \section \subsubsection \subparagraph

• LaTeX takes care of the section numbering.
• \paragraph and \subparagraph are really "\subsubsubsection" and "\subsubsubsubsection", not paragraphs.
• Including the command

  \tableofcontents

  will cause a table of contents to be included! (You may need to compile/process twice.)

N.B. Article, report, etc.; not letter.