The “Cardano Community Discord” at https://discord.gg/f8eN9XNp is a little dead.
The “IOG Technical Discord” at https://discord.gg/inputoutput is probably the most lively one. Make sure to subsribe to the sections you find interesting in the #start-here channel there.
Additionally, most projects building on Cardano have their own Discord servers or other channels. And there are specialised ones for Project Catalyst, the Cardano Improvement Proposals etc. pp.