I would've...
Definitely started my journey earlier; I started caring for my hair in my mid-teens, I would've started in my pre-teens when I relaxed for the very first time. Even though products weren't as great as they are now, I think starting earlier and basically slathering my hair in conditioner would've provided some benefit.
Kept it simple and just stuck to mostly drugstore/beauty supply products. I feel like salon products are decent but they're not worth the hype for me. (Again, products are definitely better now – especially those from the drugstore/beauty supply – but I think my hair would've reaped some sort of benefit.)
Tried to master my blow-drying technique sooner. I've only NOW gotten results that really make me happy thanks to a wide-tooth comb, paddle brush, a good blow-dryer, and great drugstore serums.
Bought multiple Tangle Teezers. I have one and I LOVE IT, waited too long to try it out.
Kept it simple and consistent – my setbacks have been caused by doing the exact opposite. My hair ≠ home remedies; all these avocado/mayonnaise/egg/honey masks are just a nightmare or provide absolutely no real benefit. Curiosity and neglect always make my hair dread.
Had a set vitamin cocktail sooner. Vitamins make my new growth so much stronger. I have to have at least a multi-vitamin, biotin, and fish oil. For extras, I like to take collagen, silica, and MSM.