My 2 cents
1. Configure trackpad settings to your liking (eg. secondary click at btm right corner, uncheck scroll direction: natural) and other system preferences as well.
2. Install Flash Player
3. Install AdblockPlus extension for Safari if it's your primary browser (only working extension for blocking youtube ads)
4. Watch mac beginner videos (eg. cmd-opt-v to paste, cmd-shift-3 to screenshot, shift-opt-cmd-v to paste without text formatting)
5. Install RightZoom (maximises windows fully)
6. Install AppCleaner (great for uninstalling programs)
7. Install The Unarchiver (great for unpacking rar files)
8. Install NoSleep (prevents system from sleeping especially useful for downloading files and running programs overnight)
9. Install ClipMenu (great free clipboard history manager)
10. BetterTouchTool (add window snap feature from windows and customise additional gestures to trackpad and mouse)
11. Install VLC, Office 2011, Spotify, Pocket, Google Drive and any other commonly used programs
PS: Some of them can be found in the Mac App Store, the rest just do a google search