Of all the parcels that can be mailed internationally, postcards are the easiest to send. The main thing to worry about is the amount and type of stamps that you need, depending on where you are sending the postcard. Different countries have different requirements, so check the regulations and requirements of the country that you are sending the postcard to. Once you have determined that, all you have to do is address and send your postcard.
Figure out the postage and rates for sending your postcard. The best way to do this, assuming you are sending your postcard from the United States, is to use the U.S. Postal Service's Postage Price Calculator. If that fails, or if you are sending it from a different country, go to the postal service of the country you are sending your postcard to--online or over the phone--in order to find out their rules and requirements for mail.
Address the postcard clearly and accurately. Make sure you put the entire zipcode (9 digits) on the card. Your address should read like this:
Apartment or Suite Number
Take the postcard to the nearest post office to make sure you get the correct stamps and the right amount of postage; it's worth the extra time to make sure you don't waste stamps and to make sure that your postcard gets where it needs to go.
Send your postcard at any mailbox or post office.
A postcard is 5 to 6 inches long and 3 to 4 inches tall. Anything larger is classified as a letter or large envelope, and will therefore cost more to send.