Above all, expect to make a few mistakes when starting a new printing process. I always buy extra materials to have on hand.
Most programs come with some sort of built-in card feature. Some popular programs include Microsoft Digital Image Suite and Microsoft Office Publisher, both of which include built-in templates for cards. All of the programs make it easy. To get started in Microsoft Digital Image Suite, for example, on the File menu click Create a Project and then choose Cards.
If you want to be more creative, you can create your own images and put them on standard card stock, which is made for printing. For example, these Avery note cards, which I like to use, are 4 1/4" x 5 1/2", which is half the size of a standard 8 1/2" x 11" page, and easily fit into a printer. I use my preferred method, which can be easily adapted to most programs. I call it "The Old-Fashioned Way," as it's really just onscreen page layout. Start by creating a standard 8 1/2" tall by 11" wide page in a landscape format using your imaging program.
Create and position your images and text on one half of the onscreen canvas. Refer to your program's displayed rulers for placement. Most programs and printers initiate printing on the left side of an onscreen landscaped page. In that case, drop your Avery note card into the printer width-wise and print. Voila! You're all set.
Of course, there's a lot of room for creativity with these kinds of cards. Find complete directions, with pictures, for making beautiful custom cards on your computer here.