Answer : The Internet has made it so easy to teach yourself a subject when you have the interest and drive. There are really just two things you need to do to learn landscape design:
1) Follow some good garden bloggers, for instance Stewart Smith at Garden Design Magazine. Make sure they're appropriate for your area because there's many different climates in the world. (For instance, this list is focused on North American garden blogs.)
2) Read their tutorials specific to landscape design-- and if they don't have one on whatever you want to know about, go find an authority and read them instead! I've compiled a list of top 5 websites with free lessons below that will answer your question more comprehensively than I can here.