Show up in a decent suit, dress shirt, and tie. Or, a blazer, dress pants (Dockers, or Khakis ok) and dress shirt, and tie. Dress shoes, shined and NO WHITE SOCKS.
Appearance counts, and look your best. In a way, it is a "second interview" with your new bosses.
SHAVE before you leave the hotel, or wherever you sleep the night before you EOD. If you have a mustache, trim it up, to short & neat. Sideburns also should go away, and the 'goatee' thing, if you have stuff like that.
Ditch the nose-rings, ear-rings, and any
other visible piercing's or decorations. Keep other jewelry
low-key, like just ONE ring per hand (HS, college, or
fraternity ring, wedding band, OK.)
Otherwise: for most pre-academy work days, keep it business or business - casual. Khakis, Dockers, Corduroys, or other 'dress' class trousers. Loafers, Oxford tie-shoes, or Cowboy boots (no tennis shoes, work boots, or other "casual" footgear.)
Dress shirt with collar, or maybe Polo or Golf style shirts. Blue chambray shirt might be nice, stuff like that. No T shirts, no tank tops, no "muscle" shirts, nothing with sports logos or used "Dominos" shirts.
Of course, dress in stuff that is neat, clean, and climate appropriate for your location and the local weather. No holes in your clothes (unless it is supposed to be there, to fit: head, arms, legs, etc.)
REMEMBER: you are a TRAINEE from DAY ONE. Dress like a young professional, and like your job depended on it. BECAUSE IT DOES!
If you show up looking like a bum on your EOD date, you will be remembered as the newbie who IS a bum. The first Agent you see in the lot, may be your FTO in 6 months. If he or she thinks you look like a slob during your first few days on the job, he/she will think that you are a slob, when you walk in 6 months later, in a uniform. That "first impression" will linger, believe me!
If and when your Station or Sector Chief
tells you to "dress down"
At the ACADEMY, use the same rules. Dress well, to show your respect for yourself, and for your superiors. If you look 'squared away' before you get your uniform, they will mentally expect you to be 'squared away' when you put your first pickle suit on.
Again: you only get one chance to make a favorable "1st impression" on your instructors. Get it right.