Finder is powerful and usually really easy to use with just keyboard shortcuts. That said, for some reason, you can't pop into the Tags menu to add tags without grabbing your mouse. Mac OS X Hints shows how to make a keyboard shortcut.
All you need to do is head into System Preferences > Keyboard > Shortcuts > App Shortcuts and hit the + key. Then, select Finder under applications, enter "Tags…" (the ellipsis is Option+;) for the title, and set your shortcut. Now, when you select any file in Finder, you can hit that shortcut to open up the Tags menu and start tagging your files quickly.
10.9: Assigning Tags by Keyboard shortcut in Finder [Mac OS X Hints]