This is more of a Linux question for Red Hat than the forum but I will try to help.
TO download Red Hat you need to purchase Red Hat, there is no other way to download the bits. Instead, you may wish to start with either Fedora or CentOS. I use CentOS personally. Go to www.centos.org and you will see a download link. You just need to download the proper ISO. From that ISO you can build a VM. THere are plenty of references on installing Linux.
Edward L. Haletky
vExpert 2009-2017, VMTN Community Moderator