I'd recommend beginning at the beginning, so Volume 1. That way, you get the full story as the author intended without missing any crucial details or build-up.
I'd suggest beginning with the first chapter. It sets the foundation and introduces the main characters and the initial plot. From there, you'll be able to follow the story's progression smoothly. Also, check out some online manga platforms that have clear starting points and good reading interfaces.
I'm not sure about legal places to read it for free with good translations. It's best to support the creators by buying or subscribing through official channels.
It depends on your personal preference. You can start immediately if you can't wait to know more. Or, you might want to wait a bit to let the anime's impact sink in.