The VE sessions at the hamfest are hosted by the Shennandoah Valley ARC. You will need to pay a fee of $15 which goes to the ARRL. You will also need to have a FRN number from the FCC if you do not have one. The Shennandoah Valley VEs would like you to pre-register for the the exam but this is not necessary. Their registration page has helpful info including how to get an FRN. Link to Shennandoah Valley registration form The url is https://www.winchesterhamtest.com
For candidates who are new there is an additional FCC $35 fee which will be billed by the FCC after the exam. This does NOT apply to license upgrades. The ARRL has announced a program to reimburse the $35 fee for applicants under the age of 18 but the reimbursement mechanism is not yet spelled out.