I do postcards in Photoshop, then bring the file into InDesign to layout to set them as two per page. I use an 80 lb cover weight paper, which will hold up in the mailing process. 100 lb cover paper is best, but I am not sure if a consumer grade printer will handle that. A copy center such as Kinkos would be able to print your file on their high quality printers. .
If you do not have access to these programs, you can use Microsoft word, or Publisher. Powerpoint works great for design, too. Photoshop Elements is a good program for photo editing, and can work as a layout program, too. I am a graphic designer so I have figured out lots of ways to do these types of projects. Good luck!