Can you recommend some good QGIS plugins?

If you work with raster files, use the Value Tool.

Can I use Python 2.7 for the homeworks?

No. Python3 is mandatory, otherwise the code we give you will not run.

How to use the “command prompt” (also called shell or terminal) under Windows?

This very handy PDF from the IHE Delft explains how to start a shell where you can access all the GDAL utilities.

Where can I download DTMs?