To print out postcards, find or create a postcard file, insert paper into the printer, and print away. Or send the postcard image files to a local print shop, and they'll take care of the rest.
To print out postcards at home, you need to have a printer with an adjustable feed tray. In other words, the spot where you insert the printer paper needs to have adjustable flaps so that adjusting to the smaller paper size is easy. This insures that the post card will print out properly. Of course, taking the images to any print job to a local printing business, and they will do it for a fee. Fees depend on the quantity of items printed, number of colors, type of paper used and requested turn over time.
Standard postcard size is 4.5" x 6". If making an original file in a creative suite, make sure to use these dimensions and set the file's resolution to three hundred dpi. This is the standard resolution setting for print media, and will result in the best-looking image once printed. Anything lower will probably appear pixelated and unprofessional. If working with a print shop, they will ask you to create a bleed on the outside of the image.