Most Important subject (both personal & professional).
Not taught but learned (eg Gandhi).
Not theory but practical.
Most important to understand the reporting structure and the “Norms for communication laid down in a company & Rules for the class ” These are not normally written down. You must read the HR Manual and the Employee Hand book thoroughly. Ask the HR to brief you in detail.
Find a buddy who has been in the organization for over 2 years and take his help to understand the internal politics and power struggles and avoid them.
Understand the etiquettes of not only speaking but also written, e mail, phone call, body language that is established.