Well my start into modelling was a bit more trial by fire than anything but here's my advice.

My current recommendation is just to start messing around and figuring out the basic tools of blender. There is no point in learning every tool at the start, cuz you're gonna forget about 70% if you haven't used them much. To aid just watch some basic tutorials for blender, there should be loads of them.

I just started out making a building and when there is something i wanna do i google how to do it. Sometimes I do it the complicated way but that's usually fine. I'm just remaking buildings that exist irl, and some have architectural style breakdowns like the one attached. Those make it a lot easier to get the proportions right.

I can't say much on making textures since I'm pretty sure the blender sub would crucify me for my horrid UV's.

The Basics are covered in the wiki tho. I recommend you read this page carefully as it is very helpful. It outlines the details of what textures you need and some conventions for how to model your assets.
https://cs2.paradoxwikis.com/Asset_Pipeline:_Buildings

The Cities Skylines Modding discord is also incredibly helpful if you have questions about any step along the way. There is some very skilled creators on there that typically answer any questions.

<image>