The following software is available for free, and a great way to get started with creating in mixed reality!
Unity 3D - A cross-platform game engine that can be used to develop 3D games and simulations. Free download for beginners and students. Learn more, and watch tutorials on their website.
Unreal Engine - A game engine initially developed for first-person shooter games, now allows you to build VR environments for a variety of hardware. You can even use their software in VR, so you’re able to build VR in VR. Free download, and tutorials/documentation available at their website.