How do I address a letter to a woman
It depends on th degree of formality and also if it's an e-mail or a paper letter.
Sz.P. is an abbreviation of Szanowna Pani, used on envelopes. It can be used in full form as a letter heading but it's very formal.
Szanowna Pani, (no name here)
It's appropriate if you don't know her in person or if it's some kind of a business letter.
If you know her and you're on first name terms, then write:
If it's an e-mail, you may start with Dzień dobry.