From the beginning you can choose episode VII
-The force Awakens-
It will ask if you want to start the episode select yes
There are five levels you must play each to complete the story mode and unlock the achievement.
To progress the story follow the 3 gold red arrows pointing down leading to diamond logo with a !.
The mission will have a blue spinning icon and when standing inside select start level. Their is other stuff to do and collect which is needed for completion. If you are new to LEGO games just enjoy and run through the story and then come back after to get all the collectibles you miss since you will need to wait to play free play to get collectibles.