How do I mark regions

In sony sionoundforge it allowed you to press the M key to mark regions for editing;supposing you wanted to delete a certain section.How do you do this function in audacity are there mouse or key functions to mark regions?

Audacity does it with “labels”. There are several ways to create labels. See here for details: http://manual.audacityteam.org/o/man/label_tracks.html