First imagine your model to be a sphere. (and your player/camera too)
As a sphere has a radius it is easy to detect collision.

However that is not verry precies. But it is a starter.
So once you detected a sphere collision it is time to move on to perhaps detect collision at triangle level. Leaving you to compare all triangles of both meshes. So for multiple part meshes it could be adiviseable to represent eache part as an sphere before going in deeper.

Also do some research on (axis aligned) bounding box collisions as they are more presice then spheres.