I would address it Dear whatever you usually call your manager. It is a formal letter but no need to suddenly call your manager Mrs Smith if you normally call her Sue.
You need to say you are notifying her of your pregnancy, your due date is X, and you currently intend to start your maternity leave on x date.
You could then add that you look forward to receiving written confirmation of the dates of your maternity leave and the pay you can expect to receive.
Your employer must write back to you confirming the date you will start your leave and telling you what date you will be expected back at work. They should also include information about what pay you will get.