The vocabulary used to write a postcard in French is quite simple, as there is not a lot of room on a postcard.
To start your postcard you can use the word "coucou" (colloquial hi) with friends or family or "salut", "bonjour" and to say good bye, you can say "à bientôt" or "au revoir", "bisous" (kisses) or "gros bisous" (big kisses). Have a look at the samples below:
Hi, I hope you are well, here, the weather is fine and hot, too good, beach every day, big kisses.
Name of the adressee(s)
Number, Name of the street
Post Code - Name of the town
Country
Dear Colleagues and Friends, I am spending a few days in Spain far from work, I hope everything is alright without me, because I don't know I will come back, so see you (perharps).