There are a number of approaches for typesetting greek with LaTeX. We recommend using UTF-8 and babel as shown in the following example.
\documentclass[12pt]{article}
\usepackage[utf8x]{inputenc}
\usepackage[greek]{babel}
\begin{document}
Γειά σου κόσμε
\end{document}
It is cleaner and more standard than other approaches using either the now old iso-8859-7 encoding, or representing greek characters with latin characters.