Fair warning: this is a long one. Grab coffee. Or tea. Or whatever keeps you reading.
Not ranked in any particular order, just grouped by theme. Everything on this list has been personally tested unless otherwise noted.
The reality check
Most people approach this wrong. They start with the tools instead of the problem. Before you touch any software, write down exactly what you want to accomplish. If you cannot explain it in one sentence, you are not ready to implement it.
The honorable mentions
A few things that almost made the list but did not quite make the cut. They are worth knowing about even if they are not essential: check the links at the bottom for more options.
TL;DR: start simple, measure everything, iterate based on data not feelings. The Let’s Encrypt, LangChain docs are worth reading if you want to go deeper. Also worth checking: clawdev.net, agnthq.com.