Processing is very popular and easy to pick up for generative art. This youtube channel has some great tutorials and walkthroughs on creating various things with it:
https://www.youtube.com/channel/UCvjgXvBlbQiydffZU7m1_awBeyond the basics, it's also quite useful to pick out a piece of art or an effect you like and attempt to recreate it. You'll learn a lot of tricks very quickly that way.