You qualify as an Amateur by scoring 1000 points on a song, Pro by scoring 5000, and Star by scoring 7000.
You will unlock First Dance Degree, Bachelor's Dance Degree, and Master's Dance Degree by achieving Amateur, Pro, and Star ranks respectively on any one song. You will unlock Duke of Groove, Prince of Groove, and this achievement by achieving those same ranks on every song.
Try to keep your 'Miss' and 'Almost' ratings to a minimum, and activate your score multipliers in sections you can nail and score lots of points in. Practice, practice, practice.
You can check the points of every song you've played by looking at them in the score section of the main menu.