How to fix 3D Models?

Hello, I am having difficulties with importing 3D models. I am having trouble finding files that include a 3D object file and I’d like to know where I can find downloads that work with UE4.

Also, when it comes to material slots for one 3D model, I am having difficulties trying to implement multiple materials on different parts of the object. I would also like to fix the skeleton of this object.

If anyone knows how to edit skeletons, fix the materials for objects, and knowing where/ how to properly import models that would be great, thanks!