Here is an example using scrlttr2:
\documentclass[
SN,% lco file for swiss letters (SN 010 130))
refline=dateleft,
firstfoot=false,
enlargefirstpage,
backaddress=false,
foldmarks=false,
parskip=half-
]{scrlttr2}
\usepackage[english]{babel}
\setkomavar{fromname}{My Name}
\setkomavar{fromaddress}{My Street \\ My city}
\setkomavar{date}{The place where I am, \today}
\setkomavar{subject}{The title of the job \quad --\quad 100\%}
\setkomafont{title}{\bfseries\normalsize\raggedright}
\makeatletter
\@setplength{firstheadwidth}{\textwidth}
\makeatother
\renewcommand\raggedsignature{\raggedright}
\usepackage{blindtext}% dummy text
\begin{document}
\begin{letter}{Compagny\\Contact person\\Street\\City}
\opening{Dear Mr. Forexample,}
\blindtext
\blindtext
\closing{Best regards}
\encl{enclosures}
\cc{other recipients}
\end{letter}
\end{document}

To shift your address a little down add the wanted shift to the pseudolength firstheadvpos.
\documentclass[
SN,% lco file for swiss letters (SN 010 130))
refline=dateleft,
firstfoot=false,
enlargefirstpage,
backaddress=false,
foldmarks=false,
parskip=half-
]{scrlttr2}
\usepackage[english]{babel}
\setkomavar{fromname}{My Name}
\setkomavar{fromaddress}{My Street \\ My city}
\setkomavar{date}{The place where I am, \today}
\setkomavar{subject}{The title of the job \quad --\quad 100\%}
\setkomafont{title}{\bfseries\normalsize\raggedright}
\makeatletter
\@setplength{firstheadwidth}{\textwidth}
\@addtoplength{firstheadvpos}{.5cm}% <- shift the head with your address .5cm down
\makeatother
\renewcommand\raggedsignature{\raggedright}
\usepackage{blindtext}% dummy text
\begin{document}
\begin{letter}{Compagny\\Contact person\\Street\\City}
\opening{Dear Mr. Forexample,}
\blindtext
\blindtext
\closing{Best regards}
\encl{enclosures}
\cc{other recipients}
\end{letter}
\end{document}

If you load visualize.loc in your preamble you can show the position and size of some fields:
\LoadLetterOption{visualize}
\showfields{head,foot,address,location,refline}
