Take a look at a real life building that you like and try to recreate it, that way you improve your building experience quick and easy.

I can recommend two servers.
GreenLeaf Creative where they have an actual "Learner" rank that basiclly gives you access to a lot of commands and a priority to be taught how to build by a Build Judge.

There's also Aliquam Creative where I'm sure you can find some great builders who can teach you how to build :D

Good building is pretty subjective. There's detail heavy styles like Grian's, https://www.youtube.com/channel/UCR9Gcq0CMm6YgTzsDxAxjOQ/ There's also the more engineered and visually angular builds of andyisyoda, https://www.youtube.com/user/andyisyoda I think these builders are both very good.

I would recommend just building something in a flat world. Try to take what you are interested in and integrate it into your build, somewhat. I'm interested in early 20th century American buildings, Medieval European buildings, and also traditional and mid to late 20th century Japanese buildings. I've been building mostly western styles, but I'm starting to have fun with Japanese.

Maybe check out my profile, there might be something you're interested in. Admittedly, my library of creations isn't numerous on PMC, but I've been building for a few years. There's also some people I'm subscribed to that do some really cool things. It took a bit of digging to find them haha.

There's many options you can take, and I would recommend looking at everything. Building on your own, getting inspiration from the real world, watching building tutorials, looking at screenshots of projects others have made, downloading some worlds, do everything. Don't let it overwhelm you, but have fun with what you're interested in.

I don't have discord, but I can recommend some building servers. Just browse, there are quite a few.