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Hm. That's a tough question, because there's so many different directions you can go with procedural generation!

A good place to start, I believe, is the PCG (Procedural Content Generation) Wiki (http://pcg.wikidot.com/). Here you can browse and gain a cursory sense of the different areas that procedural generation covers.

After you've identified an area that you like, I'd recommend diving into academic papers, blog and forum posts, and especially look at games that utilize procedural generation. Then start fiddling around in Unity or whatever. That's how I did it.

Here's some procedural generation stuff I've used in the past.

http://tinysubversions.com/spelunkyGen/ - Spelunky procedural generation

http://philipgalanter.com/generative_art/wiki/inde... - For creating tree-like stuff

http://jayelinda.com/modelling-by-numbers-part-1a/ - Procedural 3D meshes in Unity (my favorite tutorial for proc gen)

There's a lot more, obviously, and if you have an interest in a certain field than I can potentially point you in a good direction. But that's my advice for getting started.

Good luck!