There are two forms of address in Italian, formal and informal. You should alsways stick with the formal until you are on a first-name basis.

1.yes
  si
  see 
 
2. please
    per favore
    payr fa-vo-ray
 
3. don’t mention it
    prego
    pray-go
 
4. hello
    buon giorno
    bwon jor-no
 
5. good evening
    buona sera
    bwo-na say-ra
 
6. excuse me
    scusi!
    skoo-zee
 
7. I don’t understand
    non capisco
    non ka-pees-ko
 
8. no
    no
    no
 
9. thank you
    grazie
    grats-yay
 
10. that’s very kind
      molto gentile
      mol-to jen-tee-lay
 
12. goodbye
      arriverderci
      ar-ree-ve-der-chee
 
13. good night
      buona notte
      bwo-na not-tay
 
14. excuse me (to get past)
      permesso!
      per-mes-so
 
15. I don’t know
      non lo so
      non lo so
 
16. ok/that’s fine
      va bene
      va ba-nay
 
17. thanks very much
      grazie mille
      grats-yay meel-lay
 
18. hi/bye
      ciao
      chow
 
19. see you later
      a piu tardi
       a pyoo tar-dee
 
20. I am sorry
      mi scusi
       mee skoo-zee