You would do something like this to get the path "Data\ich_will.mp3" inside your application environments folder.
string fileName = "ich_will.mp3";
string path = Path.Combine(Environment.CurrentDirectory, @"Data\", fileName);
In my case it would return the following:
C:\MyProjects\Music\MusicApp\bin\Debug\Data\ich_will.mp3
I use Path.Combine and Environment.CurrentDirectory in my example. These are very useful and allows you to build a path based on the current location of your application. Path.Combine combines two or more strings to create a location, and Environment.CurrentDirectory provides you with the working directory of your application.
The working directory is not necessarily the same path as where your executable is located, but in most cases it should be, unless specified otherwise.