Student or Learner
how can i write postcarde, plz?
Welcome to the forums.
Postcards are completely informal and open to all who handle them, so they are written like any informal communication. Minimum formal start and finish.
They can be as simple as "Wish you were here" or as complex as telling the recipient what the weather is like, what the hotel/resort/swimming pool is like, and whether or not the food or drink is good.