Download an application onto your computer that can create graphics based on instructions entered by the user. Some possible applications are Processing, NodeBox, Structure Synth, Adobe Flash, vvvv: A Multipurpose Toolkit, 3D Studio Max, Blender, Cinder and Context Free. Keep in mind that these programs each require you to enter their instructions in specific programming languages. You will have the easiest time with a program that understands a programming language that you already know.
Explore blogs and websites with examples of generative art. Think about what kinds of generative art interest you and why. Develop ideas about kinds of generative art you which to make or ideas you'd like to explore by making generative art.
Start inputting your ideas into the program using commands in the appropriate programming language. If you encounter difficulties with programming the commands, visit the manual for the program you are using or download and follow an online tutorial.
Let the program run to produce your image. Initially, set the program to either save the image or display it on the computer screen. Once you create an image that you really like, you can print it or have it printed professionally.