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LaTeX will automatically indent the first line of each paragraph that doesn't immediately follow a section heading. If you'd like to get rid of an indent, you can use the \noident command:

\section{Introduction}
This is the first paragraph.
 
\noindent This is the second paragraph.

If you'd like to indent the first paragraph after a section tag, you can include the indentfirst package in your preamble:

\usepackage{indentfirst}

If you'd like to indent a whole paragraph, there are several techniques described in this post on the TeX StackExchange forum.