Where should I start?

Best idea is learning the software read there wiki, watch there youtube videos some are out of date but they will give you a really start of how to do things, this is your first step, then map out what you want to make and what you want to do, then create parts of it slowly and best idea enjoy making it.

Create A Boss Mob, Here is how.

https://www.youtube.com/watch?v=OOqwFRNfFtM&t=738s

You could start thinking little, set the basis, then expand from that. Just think of a good magic system (for example, the simplest but also most advanced one I know is the one from Electroblob's Wizardry), add the first basic spells/wands/tomes/runes/etc. and the rest will follow :)

mining ores and creating dimension portals and so on

Definitely not my mod XD

Adive tokked from life: Don't go too big on your first project, everyone thinks that they will do a greate top thing but in the end geitting overhelmed with the work or the difficulty of the idea.

This applies for both Mod Development and Game Dev

I suggest you check our tutorials collection playlist on our YouTube channel which contains many examples and tutorials that can help you get started with MCreator: https://www.youtube.com/playlist?list=PLAeL-oIFIEngE6jRgFYeFMfuj8WQsO3Ei