How do I add dots in TOC?

Try the tocloft package with something like

\usepackage{tocloft}
\renewcommand{\cftsecleader}{\cftdotfill{\cftdotsep}}

in case you're using the article class (See the package documentation for other classes).

As an illustration, let's try it with the sample2e.tex file; following pdflatex sample2e (two times), we got

One way of doing this is to use the memoir class, where this is the default behaviour.

Memoir incorporates an extensive set of functions borged from other useful packages; I use it as my day-to-day general-purpose style for almost everything except presentations.