This achievement is earned by creating an account with YourShapeCenter.com, and using it to track your progress online. If you already have a UPlay account, sign in with that instead. Be sure to link your gamertag, as you'll need your friends list for the Ready Steady Go! achievement. Once this is complete, play the game, complete one exercise, and you'll get the achievement.
If you made an account for the first Your Shape game, you will unlock this after completing your first exercise in this game.