1. Ważne polskie słowa
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2. dzień dobry/ dobry wieczór
dzień dobry (good morning) which is the most universal greeting and works fine for most of the day or
dobry wieczór (good evening), which is limited to late hours

3. Cześć - less formal greetings
cześć, which is perhaps the most popular (and least informal) of them
cześć is a useful word to use whenever you want to greet someone whom you know well or have met before and who is not significantly older than you
cześć may be a banal and everyday word but is also has quite a history, its other meaning is 'honour', which reminds us of the time when greetings were a sign of respect.
Alternatives to cześć include:
serwus (rather old-fashioned)
siema (very colloquial).

4. And now to goodbyes
cześć comes in handy again, as it is used both for hello and goodbye. However for more formal situation, be sure to use one of the following:
do widzenia, which is the most standard good-bye, or
do zobaczenia, which is slightly more formal
Both can be translated as ‘see you later’ (but literally mean 'Until we see each other again')
do jutra ’see you tomorrow’ , na razie!
But perhaps the easiest option is a simple: Pa!
If it's late and you’re heading to bed use: dobranoc (notice it’s one word, stress the penultimate syllable) – good night!

5. Proszę
proszę means please, but it's also so much more. Here's why it's perhaps the most useful Polish word you'll ever learn
When used properly proszę really opens every door and melts everybody's hearts. Using proszę is really simple, especially since this universal word works like a skeleton key and can be used in many social situations which in English normally require very different phrases. Compare the following phrases and situations:

6. Proszę Excuse me? (to ask someone to repeat themselves)
Go ahead! (when you let someone go ahead of you – in a doorway, etc.)
Here you are! (when handing something requested)
Not at all! (as in when someone says ‘thank you’ and you answer ‘no problem’)
Well, well! (when irritated)
Come in! (when answering the door)
Please! (when asking or pleading with someone)
Hello! (what you say when you answer the telephone)
In all of the above situations you can comfortably say proszę - it covers them all

7. Przepraszam
used both in situations when you want to approach someone, say, in the street (e.g. ‘Excuse me, but can you tell me what time it is…’)
and when you mean to apologize to someone for something (‘I’m sorry’)
Again, one word covers those two quite different situations

8. Dziękuję Last but not least, remember dziękuję
This is plain ‘thank you’
or dzięki ‘thanks’ (less formal)