\documentclass[pdftex, 12pt]{scrlttr2}

\usepackage[margin=2cm,pdftex,a4paper]{geometry}
\usepackage{lipsum}

\setkomavar{fromname}{Donald Duck}
\setkomavar{fromaddress}{123 Sesame St\\London\\United Kingdom}

\begin{document}

\begin{letter}{%
    Ms. Piggy\\
    123 Sesame St\\
    London\\
    United Kingdom}

\opening{Dear Ms.\,Piggy,}

\lipsum[1]

\closing{Sincerely,}
\end{letter}
\end{document}
