\documentclass[12pt]{article}
\usepackage {ulem}
\usepackage [utf8]{inputenc}
\usepackage [greek,english]{babel}
\usepackage {alphabeta}
\usepackage {amsmath}
\usepackage {amssymb}
\usepackage {tabularx}
\author{Ειρήνη}
\title{My first title}
\begin{document}
\maketitle
\section{My first section}
My first paragraph η πρώτη μου παράγραφος $\sum_{i=1}^{n} x_{i} = K$
Εξίσωση σε ξεχωριστή γραμμή χωρις αρίθμηση
$$
	\sum_{i=1}^{n} x_{i}
$$
και αν θέλουμε αρίθμηση στην εξίσωση ~\eqref{eq:1}.
\begin {equation} \label{eq:1}
\sum_{i=1}^n x_{i}
\end{equation}
\subsection {My first subsection} \label{s1}
\begin{center}
``LaTeX is a \underline{document} \textbf{preparation} \sout{system} \dotuline{and document} \emph{markup language}. 
\uwave{LaTeX} uses the TeX \emph{typesystem} program for
formating its output, and is itself written in TeX macro language''.\\
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 θα δμιουργήσουμε τον πίνακα ~\ref{tbl:1} που είναι στην ενότητα ~\ref{s1}
\begin {table} [ht!]
\centering
\caption{Ο Πρώτος μου πίνακας}\label{tbl:1}
\begin{tabular}{|m{2cm}|m{8cm}|m{4cm}|}
\hline
\textbf {Heading 1} & \textbf {Heading 2} & \textbf{Heading 3} \\ \hline
test1 & test2 & test3 \\ \hline
v1 & v2 & v3 \\ \hline

\end{tabular}
\end {table}

\end{document}

