The Book of Shaders [1] perhaps? It's about fragment shaders rather than geometry but it felt to me like a good introduction to generating things programmatically.

[1]: https://thebookofshaders.com/

Start following the relevant communities like old.reddit.com/r/proceduralgeneration/ and #procgen tag on social networks, read up on techniques others use. Commit to jams and community events such as PROCJAM and GENUARY.

Learn about perlin/simplex noise, Poisson disk sampling, packing algorithms, WFC, boids, inverse kinematics, SDFs.

Choose a style (2D, 3D, animation...) and from that start learning the relevant tool (Blender nodes, Houdini, Processing or something else). This is a decent start and it branches too much from here on, depending on your tastes.

The coding train YouTube channel is also a great place to start, its based on processing.js but honestly most procgen techniques can be applied in any language