Excellent advice from all three above posts.
To add to DavidL's advice, find a few cheap but nice items at flea markets, ebay, or even lying around the house and study how they are put together, or even take them apart to learn how they are put together. Ask yourself why the maker made each of their design decision. Was it an engineering decision, aesthetic decision, or cost/time saving. Analyzing a few items that way will tell you a LOT.
Consider finding a few patterns for free online, or buy one or two to see what others have done, and again, analyze their decisions. Once you understand this sort of thing, your own designs will come far easier.
I agree with Jim, nothing beats hand-drawing designs for learning how to do it. Graph paper is a big help in hand-drawing and you can find it just about anywhere.
When you are ready to do some computer based design, you'll find that there are a lot of advantages. Computer based designs can be done for cutting or carving. They are easily filed and organized without taking up physical space. They are perfectly repeated with every printing. They allow a level of precision that hand-drawing does not, although that is not always important.
If you have a carving pattern, you can make a fresh pattern every time you want to use it by printing onto paper, or even better printer transparency film. By printing a fresh copy every time you want to carve that pattern, it prevents distortions from creeping into your work with repeated use of the same pattern.
If you have a cutting pattern, there are a few options. You can trace the pattern into the leather to give you cut lines, much the same as a carving pattern. If you will use your pattern a few times, you can print it and then glue it onto thick cardboard, cut it out and use it a few times. George Hurst did a youtube video on this for Tandyleatherfactory. With time cardboard patterns can wear, but they are good for some things and limited use. Cardboard patterns can, of course, be remade from your original when they wear out, but that is an investment of your time. If you think you will use a pattern many times, it's worth investing in a laser cut plastic pattern. I've been very pleased with the work and pricing from www.blackriverlaser.com . The owner, Joyce is delightful to work with.
Hi Im Joe suggested Inkscape, which is will probably do nicely, and the price is right! The important part, though, is Vector Based. It's the only good way to get nice smooth lines. Avoid pixel based or raster based software. The differences are much too technical to get into here, but trust us on this one.
I managed to get an antiquated version of Adobe Illustrator that does all that I need it to do. If you go with Illustrator, there is an excellent tutorial in 19 parts on youtube by TastyTuts. There is a link to sample files which allows you to follow along with the videos in your own copy of Illustrator. There is not better way to learn than to do!! The videos are not leather-work oriented, and later parts not-so-applicable to what we do, but I watched them anyway and filed them away in my head as "good to know". When you are ready to take your earlier hand drawn patterns into the digital realm, you'll need a scanner. They're pretty cheap these days and useful for a number of other uses as well. I bought an all-in-one printer/copier/scanner and love it. You'll find the above-mentioned tutorials 13 - 16 useful.
You can also find a short youtube video by Ian Atkinson on how to take a simple hand-drawn design into Illustrator, although it is a little difficult to follow until you've learned a bit more about Illustrator. The general principals should apply to any software you choose to use.
PS. Jim. I'd love to take a look at your book! Where can we find it?
I hope that helps more than confuses!
Bill