Formal:
* Miło mi Pana/Pani poznać. (Mee-loh mee Pa-na/Pa-nee po-znach) - This is the most formal and polite way to say it, literally meaning "It's nice for me to meet you."
* Miło mi, że się poznaliśmy/poznaliśmy się. (Mee-loh mee, zhe sheh po-zna-lee-shmee/po-zna-lee-shmee sheh) - This translates to "It's nice that we met."
Informal:
* Miło Cię poznać. (Mee-loh tsee po-znach) - This is the most common way to say it in informal settings, meaning "Nice to meet you."
* Miło, że się poznaliśmy/poznaliśmy się. (Mee-loh, zhe sheh po-zna-lee-shmee/po-zna-lee-shmee sheh) - Similar to the formal version, but more casual.
Note:
* Pan/Pani (Pa-na/Pa-nee) is used for Mr./Ms. and is the most formal way to address someone.
* Cię (tsee) is used when addressing someone informally, like a friend.
* Poznaliśmy (po-zna-lee-shmee) is the masculine plural form, while poznaliśmy się (po-zna-lee-shmee sheh) is the more neutral form.
You can also simply say "Cześć!" (Chesh!) which means "Hello!" or "Hi!" This is perfectly fine in informal settings.