Mathematica
General information
General information on the usage of Mathematica can be found at the Wolfram Documentation Center.
Export
Information on how to export data in different formats from Mathematica can be found here:
https://reference.wolfram.com/language/ref/Export.html?q=Export
"How can I convert a .nb-file into a .m-file?"
To convert a .nb-file into a .m-file, you can open the .nb-file on Mathematica, select the lines which are supposed to appear in the .m-file and flag them as a Initialization Cell via Cell -> Cell Properties.
Now save the file (File -> Save As... ) and choose the format "Wolfram Mathematica Package (*.m)".
The Initialization Cell is then exported as a comment/text.