It depends on quantity of people. If You talk to one person of same age - You should say: Здравствуй! Дай!
If You talk with 2 or more persons You should say: Здравствуйте! Дайте!
And if You talk with one senior person (or not senior but not familiar to You) you also should say: Здравствуйте! Дайте!
This form - "-те" used for multiple persons or like polite form for unfamiliar or senior persons.
There is 2 translations of "You" in Russian "ты" and "вы". "ты" - for friends, relatives and other people that familiar to You "вы" - for unfamaliar and senior people and also for multiple persons
Ending "-те" used for those persons, which we call "вы"