If you want to suppress indentation for a single paragraph, you can use
\noindent.
This should be very rarely used; most of the time, you should let LaTeX
handle this automatically.
\documentclass{article}
\usepackage[T1]{fontenc}
\begin{document}
One small paragraph, which we have filled out a little to make sure you can
see the effect here!
One small paragraph, which we have filled out a little to make sure you can
see the effect here!
\noindent One small paragraph, which we have filled out a little to make sure
you can see the effect here!
\end{document}