...don't even think about letting down your hair!
Nope, don't do it. Only after your shower and when your MTI is long gone! There's a number of sources for information around the web about how to put your hair up in a sock bun, so I won't dwell on that too much.
Gentlemen, I don't have much to tell you. Make wise decisions though. Chop off any long hair or mohawks before you head to Lackland, but don't go as far as shaving it all off. Let the folks at Clipper Cuts take care of you. Don't show up with anything carved into the side of your head. You want to draw as little attention to yourself as possible, in terms of outlandishness.
Ladies, oh ladies! Your hair is about to go through eight and a half weeks of stress and torture. After I finished BMT, all I wanted was a deep conditioning treatment. Heat damage won't be the issue, as you won't be using a hairdryer during BMT, nor will you be using conditioner.
To my black/women of color readers - err on the side of caution if choosing to shell out the money and get your hair braided (1/2" or less) before you go. It's up to your MTI team's discretion if you'd have to remove them or not. There was never a lot of clear guidance, as you'd typically get an OK from one MTI and catch hell from another. Due to the differences in reactions from MTI to MTI, many of my flight members chose to just take their braids out, including micro braids, as to not have to deal with being hassled. Near the very end of our training, a few girls were allowed to purchase relaxers, but again, this is at the discretion of your MTI.
It is possible for you to get your hair cut as a woman at BMT, but typically only if your short hair isn't short enough (it's touching your collar). Beauty shop runs were a very rare occurrence, and again, they were at the discretion of our MTI and if our training week allowed for it.
For the record and for your own comparison, I have stick-straight, thin hair, but a lot of it. Before I left for BMT, I practiced my sock bun with my Conair Bun Maker and I thought I was pretty good. I'd need to get faster, but I thought I wasn't in too bad of shape.
Down at BMT, I was blasted time and time again in the first two weeks for hair-related nonsense. As a Dorm Chief, I would get yelled at for how the hair of the other females appeared. We had a female MTI do a hair demonstration on one of the first days, assisted by older female trainees, but it took us some time before we really took that job seriously. Eventually we started carrying around extra bobby pins in our ID card holders. While waiting to leave the dorm, or waiting to enter the chow hall, we'd inspect the hair of our wingmen and fix it as needed. If your wingman looked like a dirtbag, they'd ask you why you let them leave the dorm like that.
The key to BMT hair success is excess. Like they say, "Everything's Bigger in Texas," including the amount of product and the number of bobby pins you should be using. Many of us used Got 2b Glued, which is available at the mini-mall. The others used the big, generic bottles of colored gel. Get your hair wet and slick this stuff through your hair, pulling it back into a ponytail in the middle of your head. Put your sock bun or Conair Bun Maker on around your ponytail. Lean forward and smooth your hair around the sock bun, covering it completely. Smear more gel all over your bun. Secure with a larger hair tie. Twist the tails of your hair, working around your bun, then secure with another large hair tie. Now, pin the crap out of your bun and loose ends. I typically use 10+ hair pins, easily. Finally, spray your entire head with hairspray or spray gel and make sure any remaining flyaways are secured with bobby pins. Severity is key. You're not going for a "soft" look with this bun, so get those Pinterest images out of your head. Think Robert Palmer girls.
Doing your hair like this takes some getting used to, but you'll improve and you'll get faster out of pure necessity. I loved doing my hair after a shower, but you weren't supposed to keep your hair in a bun while you slept. We'd heard nightmare stories about trainees growing mold in their hair because they didn't ever wash it and allow it to dry out completely. I learned to do it quickly every morning and I got so good at it that it would last through morning PT with only a minimal touchup afterward when I put on my ABUs. It was very common for trainees to get red bumps at the nape of their neck. You can alleviate the situation by taking hand sanitzer (you'll apply this before every meal via dispensers) and patting the back of your neck with it.
Good luck, goldilocks! I'm still waiting for my deep conditioning salon oil treatment!