Get OpenXT
OpenXT is not just a product, it is a platform. The project itself can be quite daunting with its myriad of repositories. The main open source project is here on Github:
https://github.com/OpenXT
For some the best way to get started might be to just grab a build off this public builder. It runs nightly builds of the master branch of OpenXT:
http://openxt.ainfosec.com/builds/master/
In particular you will want the installation ISO. So e.g. for a particular build:
http://openxt.ainfosec.com/builds/master/oxt-dev-39-master/iso/installer.iso
The server runs instances of Buildbot. You can see what the main builder is currently building using the first link on:
http://openxt.ainfosec.com
To get an idea what sort of hardware you will need to run OpenXT, see the Hardware Guide to get an idea. After installing OpenXT, a guest can be created using the Install VM button at the top of the UI. The simplest way to install a guest is to create one with OS installation media (e.g. a CDROM) in the drive. Once a guest is created and running, the xctools.iso will be present in the guest to allow OpenXT guest tools to be installed. For Windows guests, the build machine certificate will need to be installed. It can be found here:
http://openxt.ainfosec.com/certificates/windows/
Follow the instruction for Installing the Windows driver certificate before running the installation on Windows.
If you want to jump in and start building OpenXT then How to build OpenXT is where you want to go.
Contribute
Contributing to OpenXT will be a bit awkward while we're establishing our process. Currently we're transitioning from a closed to an open source model so we don't yet have a handle on all of the new tools that we're using. For now use the How to contribute page as a "best practices" guide to get you started. Another good resource is the task list in What needs doing? if you want to know what needs doing on the OpenXT project.