We currently support a 3D model format called GLB (which is the binary format of GLTF). Most 3D models can be converted into this format using an online conversion tool. We're working on creating our own tool to make this even easier for you, but in the meantime we suggest the following site: https://blackthread.io/gltf-converter/
To use the converter, scroll down the page:
A preview of the original model will appear on the left, and the conversion to GLB on the right. This is really helpful for checking if you're happy with the model (this is an example created by Francis Chen that I downloaded from Google Poly in OBJ format).
Once you have your .GLB, simply navigate to the Plan area of the ClassVR Portal.