How do I know what bit my horse needs?

Excuse me, but I'm a bit confused about the needs of my horse regarding bits. Could you please clarify for me? How can I accurately determine what type of bit would be most suitable for my horse? Are there specific factors I should consider, such as their breed, size, or temperament? Additionally, how often should I reassess my horse's bit to ensure it's still the right fit for them? Any advice or insights you could provide would be greatly appreciated.