There are lots of different 3D Programs you can use. Some are simple, some are complex. Some need to be installed on your computer, others are online, try out 3-4 and find out which you prefer:
Education.minecraft.net
3D printing World -
Go to: http://bit.ly/MC3D2019
Download the size of printer you want
Import it into Minecraft
Create your own model, save & export it as an .GLB for your webVR world (or .STL to print)