Greetings and Welcome,
Currently not so much uptodate documentation is available on creating something from scratch within here, after all we are still within Alpha with things changing constantly, and there are not that many content creators, but whom are sorely needed to test out the tools already available or being development (even if it is occasionally frustrating):
However we can start with this document
So far, content creation within High Fidelity is done through importing of 3D models in FBX or obj format: Any 3D suite should suffice in creation of static objects.
The Pipeline is pretty much same as within game engines as of the moment, except importing models is done via http protocol (an alternative, peer-distributed asset hosting solution is being worked on and is already available however)
You can use tools like Maya, Max, Sketchup, Blender to make these, as long as the tools export with fbx… But you need somewhere to host these files .
The inworld edit tools are mostly used to move the objects into their locations within the world.
There also is an in-works inworld-content creation tool such as polyvoxels, but those are still under development as well. These can be used to create Terrain, and objects if you are determined enough to experiment with in-development tools.
Collision hulls have to be hulls which consist convex objects: There is a tutorial for blender available:
Avatar creation uses the same processes as in game development, however there are some specifics to High Fidelity, mostly to do with Skeleton structure and rotation. You may find technical details here: https://alphas.highfidelity.io/t/avatars-standard-compendium/6200 ¨
Downside to this is that as of the moment it require the use of one 3D suites to create or currently modify the avatar, but there are plans to implement avatar customization as far as I know: Blender currently is one which is known to have issues with avatars however, but its being also worked on (again everything is a work in progress)
Hopefully that helps