\documentclass[fontsize=12pt, paper=a4]{scrlttr2}

% Dont forget to read the KOMA-Script documentation, scrguien.pdf

\setkomavar{fromname}{} % your name
\setkomavar{fromaddress}{Address \\ of \\ Sender}

\setkomavar{signature}{} % printed after the \closing
\renewcommand{\raggedsignature}{\raggedright} % make the signature ragged right

\setkomavar{subject}{} % subject of the letter

\begin{document}
\begin{letter}{Name and \\ Address \\ of \\ Recipient}

\opening{}  % eg. Hello



\closing{} %eg. Regards

\end{letter}
\end{document}