\documentclass[12pt]{article}
\usepackage[utf8]{inputenc}
\usepackage[greek,english]{babel}
\usepackage{alphabeta}
\title {My first title}
\author {Konstantina Tsimpouka}
\date {}
\begin{document}
\maketitle
\section{My first section}
\par My first paragraph
\subsection{My first subsection}
\begin{center}
"Latex is a document preparation system and document markup language."\\
Εδώ γράφω ελληνικά
\end{center}
\end{document}
