The "M" key shortcut not working in AE

If you have not added a mask to a layer pressing the "m" key will do nothing. 

Try selecting the pen tool (g) and adding a mask, then press Alt/Option + m to set a keyframe for the mask and open the mask property. Pressing 'mm' will open all mask properties if there is a mask. 

If you are new to AE I strongly suggest that you spend about 20 minutes in the Learn Workspace getting to know the UI. You should also spend a significant amount of time with the User Guide. It will give you a head start on using the application.

You also need to know that most of the tutorials that people have problems with were found by searching the web, and most of them are pretty inefficient recipes for specific "cool effects" that enthusiasts have posted. The workflows are often poorly thought out and the explanations lack clarity. Make sure you vet your trainers. Make sure they know what they are talking about. The number of views is no indication of the quality of the tutorial. 

If you are having problems with a specific tutorial be sure and include a link to the tutorial so we can figure out what you are trying to do.

Also to add, when you select the Mask (like Mask 1) in your layer properties, your mask in the Composition panel will become selected.

Any masks attached to the layer can be found by opening Mask properties of the layer:

(you can also press M twice on the leyboard)