That's a very a big subject. I would just give it a go! Pick a product - cards are a good starting point , and relatively cheap to print - and download a template from somewhere like moo.com or vistaprint.com. And then play around with your designs.

You can draw stuff directly in programs like Photoshop, Illustrator, Inkscape etc. or scan in your own drawings.

Try mixing sizes, colours, fonts etc. and keep doing variations until it works. Have a look on sites like Folksy and Etsy to see how other designers lay out their cards. Don't copy obviously, but you should start to see what works and what you like.

If you want to do something more handmade, you can get your designs made into rubber stamps, or you could buy a gocco printer or sign up for some printmaking classes.

Another good idea is to enter design contests or start a Zazzle store - both will restrict you in certain ways (size, colours, theme) so you'll learn how to make your designs work for different products, seasons etc. Plus the feedback will help you see what's popular.

There's a LOT to learn really - I've been designing and drawing stuff for nearly 20 years now and I still feel like I have loads to learn. If you want to ask any follow up questions, I'm happy to try and help!