The most useful shortcut keys on Mac is the one for Expose and Dashboard so if you still haven’t configured them on your keyboard (or mouse) go to System Preferences ->Dashboard & Expose and configure them.

This is the first set of the 10 most useful shortcut keys on Mac that you really need to get a hang of to be productive in everyday use. 

• shift + apple key (command) + delete – empty trash
• ctrl + n – new mail window (in Mail application)
• shift + apple key + d – send mail
• apple key + w – close active window
• shift + apple key + left/right arrow – tab navigation in Safari browser
• mouse drag and drop + hold option – copy file
• mouse drag and drop + hold option + apple – create alias (shortcut)
• apple + space – on/off spotlight (search)
• apple + tab – switch to another active app
• apple + q – quit active application