Hi Endy, I’d recommend starting simple first and working your way up!
It seems like you’d like to do an ENTIRE avatar from scratch. But as I’m sure you’ve noticed, avatars are made up of lots of moving parts.
So I’d suggest focusing on the aspects that are you most interested in! For example, if you’d just like to have an avatar to customize, you could use a model made by someone else. Silent has a great list of free models here: https://gitlab.com/s-ilent/SCSS/-/wikis/Other/Avatar-Models
This way, you could skip learning Photoshop or Blender - for now. As soon as you’re comfortable with how Unity works, you could dip your toe into customizing individual parts of it, and learning third party tools.
Don’t be discouraged if it seems difficult. We all have to start somewhere, and it’s best to take things one step at a time. It’s the only way you’ll improve.
Hope this helps!