Blender and ‘Sketchup make’ are good free programs. If your on a Mac Logic isn’t free but is an amazing audio program. Blender has loads of tutorials but took me a while to get to know. Google Andrew Price or Blender Guru to get his youtube tutorials. Though Blender tutorials have a habit of going out go date as new versions of blender come out. Blendr is good for modelling. You sculpt organically with it in sculpt mode. Sketchup is by far the easiest way to get simple geometric shapes made. It also has the 3D warehouse https://3dwarehouse.sketchup.com/?hl=en where you can download just about the whole universe for free 
I use Sketchup importer https://github.com/martijnberger/pyslapi/releases/tag/0.13to import all the 3D warehouse stuff into blender, but haven’t found a way to export them from blender into Ureal Engine with their textures. But the shapes come in and I add materials to those. I think the texture export is doable I’m just to stupid
You can get free models on lots sites as fbx and obj. the 2 file types supported for import in Unreal engine.
I find all of the programs I use quite challenging. It seems like you press one wrong button by accident and your screwed until you can find out what you’ve done. That’s the minefield that I step onto anyway