For this achievement, you will need to reach Rank 20, which requires a total of 1140 experience points. Experience is awarded for doing various things throughout the game, such as correctly branching questions, gathering collectables, etc. Below is a list of all actions which reward you with experience, and how much each action gives you. Note that you will only get the experience for correctly branching a question if you don't use intuition points, and if you are on your first playthrough of the case.
Correctly branching a question in an interview: +5 experience
Completing a street crime: +15 experience
Discovering a landmark: +5 experience
Collecting a golden film reel: +5 experience
Collecting a golden record: +5 experience
Collecting a police badge: +5 experience
Collecting a novel: +5 experience
If I missed anything, please let me know in the comments.