In my opinion, the best way to prepare for a certification is to actively use AutoCAD for a certain amount of time in a 'work' environment. I believe the Autodesk recommendation is a minimum of 400 hours of experience. But depending on the variety of work you do within AutoCAD I would say that number can be a lot higher.
I don't know how it is where you are living, but here a certification is not a necessity for an entry level position as a CAD designer/drafter. Usually intake interviews are also accompagnied with some for of practical test where they see your skills at work. So the best way to 'get good' is to 'get practicing'.
