Take a look at
System.IO.Path[
^] class, it performs operations on string instances that contain file or directory path information.
To read/write a file, that is located where your program runs, you can uses something like this:
string fileToRead = System.IO.Path.GetDirectoryName(Application.ExecutablePath) + @"\MyOtherFile.xml";
string allDataFromFile = System.IO.File.ReadAllText(fileToRead);