I want to write Greek language text. How do I do this?

You can typeset Greek text with the babel package, as demonstrated by the following example: 

\documentclass[a4paper]{article}
\usepackage[english,greek]{babel}
\begin{document}

\section{Εισαγωγή}

Σήμερα είδα για πρώτη φορά το
\selectlanguage{english} site
\selectlanguage{greek}του Γιάννη.

\[
a = b + \gamma
\]
\end{document}

 Open this example in Overleaf

This example produces the following output:

Greek text typeset using LaTeX

See also the Overleaf help article on typesetting Greek text.