There are so many achievements. I just don't know what to work on. Any suggestions?

well that depends.

What do you want? Lots of achievements fast? To work toward a long term goal? To get into the blue building?

Do you have lots of sP floating around, or would you rather work on cheaper achievements?

Do you like games, collections, battle, questing, wardrobe?

Do you want to work on several achievements simultaneously? Or do you prefer to work on them individually?