I recently went through something similar for Luminant at Comanche Peak. My advice to you is, wear a suit for the test and interview. Think about it this way, every time they see you, whether for the test, interview, site visit, whatever, they are forming an opinion of you. Make the absolute best impression you can, every chance you get. In my situation, I knew the test and interview would be same day, so I wore my best. Black suit, white dress shirt, red "power" tie, Matching black shoes and belt. I made sure to wear the suit several times prior to the interview to make sure I was comfortable in it. Pick up an ASVAB test book either at the Library or B&N. take as many practice tests as you can, as fast as you can. Read the other threads about how to prepare for the test and interview. If you have time, prepare for the STAR interview questions. Hope this helps.
Good Luck
Have a Day
-Matt