>>302593046
To start, follow the official vrchat instructions to get the SDK set up, pull some model off of booth or vrcmods that's ready to go, upload it. You'll learn a lot about the general process. Then try making small tweaks. Customize the overrides a bit. Edit the texture or shaders to change the hair color. Drag in some simple prop like a button or hat.

Then download blender, and start customizing the model itself. Try editing the proportions if you have FBT. make a custom shapekey facial expression. Swap the head on top of some other body/costume. Then try fitting clothes to the original body (or other nude base). Try modeling your own simple clothes. 

Then learn anatomy, box modeling or sculpting, draw the rest of the owl. I'm not at this stage yet, but that's the next step.

>want to make something completely "Mine" from the get-go. 
I recommend instead starting with small modifications of existing models. it'll be more incrementally rewarding. If you want something that's arguably more "yours" than a random booth or vrcmods, you could start with vroid, and move the sliders around, export to vrm, import to blender, export to FBX, etc etc. I'd still recommend doing a straight no-modifications upload first though as an exercise in unity, even if you never use the uploaded model.