\documentclass[a4paper]{scrlttr2}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[ngerman]{babel}
\usepackage{url}
\usepackage{xcolor}
\newcommand{\todo}[1]{\textbf{\textcolor{red}{(TODO: #1)}}}
\renewcommand{\familydefault}{\sfdefault}
\usepackage{helvet}
\setkomavar{fromname}{Tim Buchwaldt}
\setkomavar{fromaddress}{Lukasstr. 16a\\50823 Köln}
\setkomavar{fromphone}{+49 151 16914796}

\begin{document}
\begin{letter}{\todo{Add recipient adress}}

\KOMAoptions{fromphone=true,fromfax=false}
\setkomavar{subject}{\todo{Add Title}}
\opening{Sehr geehrte Damen und Herren,}

\todo{Add text}

\closing{Mit freundlichem Gruß,}
\end{letter}
\end{document}