Take a look at this link VMware NSX Training and Certification and you will find a lot of training resources, including free webinar.
Actually there no need to previous training but consider at least have proper knowledge on basic routing and switching, and some TCP/IP and SDN stuff, check this links:
and of course VMware hypervisor and CCNA, use the subnneting workbook as well, this can help you a lot.
consider two flavours of NSX one for VMware purely, and the other One for any hypervisor, the most exciting one is Openstack, these flavours have a couple of differences on behaviour.
hope this helps.