The layout is probably the hardest part of making a greeting card, especially if you are printing an "inside" message. This link is a simple layout for a one-sided (Right Click to Download) (1/2) x 7" x 10" Greeting Card For our demo, here's the (Right Click to Download) lily painting
There are many "paper" companies, one that I like is "Red River Paper." They also have some video tutorials for Photoshop Elements and Photoshop on how to set up printing guides for Canon, Epson and HP printers. You'll find that web page here.
About.com has an excellent step-by-step tutorial on how to set up and make a 4 sided card.