I'm going to answer in two parts due to the two main features of track making for BNG.

The track-creator inside the game is used to create the racing circuit itself (no scenery). It's not hard to use, but it's also not based on preset "blocks" that you place around. There's a defined method for building tracks correctly that requires a bit of learning and trial and error. Think of it as drawing your track with a pencil on paper, but with controls over track width, tilt, etc.
There's a bit of documentation you can use to learn the basic controls.


The Unity Tools are what's used to make a distributable custom track. You feed the track files from the track creator (*.TRM) (and possibly scenery made in 3rd party software) into it and export them as a custom track. They require begginer's level of Unity3D knowledge. If you want to make scenery for your track you will need to know how to use a 3D CAD (computer aided design) such as Blender3D (free) and an image editor such as Photoshop or GIMP (free), as Unity3D's purpose is not to make 3D assets nor textures. That is another set of skills you'll need to learn, although we will provide video tutorials on how to make a simple track from scratch (track-creator included), going through scenery model and texturing, sometime in the future.


For an absolute noob in all these aspects, the best one can expect in a reasonable timeframe is just making the track circuit with the track creator and using Unity to export it to a custom track to be distributed on the steam workshop. No scenery whatsoever as that is the most time consuming and most skill demanding part of track making.