VTSP = VMware Technical Sales Professional
A VTSP is a partner-only accreditation, usually taken only by technical sales people - most commonly referred to as "pre-sales" or a "systems engineer".
There is a VTSP Foundation, plus separate VTSPs for each defined competency area.
If you don't know much about NSX-T it won't do you any harm to learn the basics via the VTSP before you take your full class in August (presumably the NSX-T: Install Configure Manage class)
If you are a partner, you should get access to the VTSP training through Partner University.
If that is not the case, you or whoever in your company manages your partner relationship with VMware should get in touch with Partner Support.