Find classes to take at your local community college because many colleges will offer a course in landscape design that will show you how to use CAD software. If you do not have time, you can look for a night or online class.
Purchase CAD software and begin to use it. Because CAD software can be expensive, you may not want to buy anything until you are sure it is something that interests you. Softwares that are common include DynaScape and AutoCad.
Search in your local library or bookstore for books or informational pamphlets that instruct you how to use CAD software, such as "AutoCAD for Dummies."
Search online for portfolios of landscape designers. Using search engines such as Google you can begin to gain inspiration and ideas. You will also be able to see what professional landscape CAD drawings look like.