Definitely get a copy of the free software VCVRack and use it to try various combination of builds. There are free tutorials on modular setups with it online. Perhaps start with a semi-modular like Moog Mother 32? That at least would get you started with a patch synth and understand the basics. Also, get a larger case than you think will be needed. Most of modular is support type modules. In my setup, I only have 1-2 voices plus a filter and LFO but lots of support tools like VCAs, envelope generator, attenuator, mixers, logic modules and so forth. These are what make modular way different than say a traditional hardware synth or you could get a Moog Matriarch that has 90 patch points and would teach you how the various parts fit together. If you really want a full modular system, another choice are prebuilt systems like Make Noise Shared System, Doepfer A100 Basic System, or Erica Synths Black System or ALM Busy Circuits Super Coupe modular system that have all the basic modules together for you.
I started first with a Make Noise 0-coast and Korg SQ-1 sequencer then VCV Rack then a Doepfer A100 Basic System and added to it over time.