In short, press Esc to get out of insert mode, and then do :wq to write the file and quit.

But in general you don't have to use vi - when you see it in instructions on the web, use nano (edit in terminal) or gedit (GUI editor) instead. When you typed:

vi common.mak

You were launching the vi editor to edit the file common.mak. Instead you could do:

nano common.mak

or

gedit common.mak

to edit the file in an editor that doesn't require learning to do simple things like saving a file.

Or if you want to learn vi then type vimtutor at the command line - that will get you to the point where you can be reasonably efficient with vi. If you find you like vi and want to learn lots more then read this post.