How can I indent the paragraphs which follow a heading?

The easiest method is to call

\usepackage{indentfirst}

in your preamble. This package is included in all LaTeX distributions.

The standard setting of LaTeX is to suppress indentation in the first paragraph after a sectional title, which is standard usage in American typography.

Some babel language settings do similarly to indentfirst, for instance the French language definition file does, to set up the document according to French typographic traditions.

The first paragraph after a section heading is not indented by default in most document classes (or rather its indentation is removed, which is why \indent doesn't help you either). If you do like all your paragraphs indented then the correct thing to do is to change this default. There is a package called indentfirst that precisely does this for you.

There are a couple of other packages that may influence/change the default behavior of the document class, e.g., titlesec has an option to specify how the behavior should be and the frenchb option for babel automatically adds indentation as this is a common typographically rule for French documents. And there may be others.

If for some (strange) reason you only want a single paragraph indented then try \indent\indent, the first will be swallowed but the second should survive.

You could redefine the internal macro \@afterheading for setting \@afterindent to true:

\documentclass{book}
\makeatletter
\let\orig@afterheading\@afterheading
\def\@afterheading{%
   \@afterindenttrue
  \orig@afterheading}
\makeatother
\begin{document}
\chapter{One}
text
\section{one}
text
\end{document}

This works without an additional package. Perhaps consider using the titlesec package which is very useful for customizing format and spacing of headings. Its command \titlespacing can make an indentation, but if used with a star such as \titlespacing* the indentation is removed.