\documentclass[12pt]{article}
\usepackage[utf8]{inputenc}
\usepackage[greek,english]{babel}
\usepackage{alphabeta}
\author {Vasiliki Faloutsou}
\title {My first title}
\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.
      LaTex uses the TeX typesetting program for formatt.LaTex is not the name
      of a particular editing program, but \\ 
      refers to the encoding or tagging conventions that are used in LaTex
      documents.
\end{center}

Εδώ γράφω Ελληνικά
\end{document}      
