I have relatively straight, untreated, fine hair. Not sure what my official hair type is.
I wish I could post a photo because it's honestly crazy how much better my hair looks now than compared to 5-6 years ago, lol. I dyed and bleached my hair for about 10 years and did it incorrectly. Never was told about proper hair care, washed it twice a day without caring about the products I was using, blew it dry almost every day, straightened it often and never used a heat-spray, sometimes even straightened it wet. I'm cringing just writing this. My hair was like straw and I barely had any hair. It was also incredibly frizzy and unruly. I had waves and some parts of my hair were really curly. I just thought it was my hair pattern because my mother had similar hair, so must have gotten bad hair genes. Now, I know it's just because I wasn't taking care of my hair. My mother still does not take care of her hair and just says "well, it must be my genes." Right.
Anyways, what I did/do for better hair:
+ if your hair is damaged, you can't fix it. You can use certain products on it to make it look better, but once it's damaged, it's damaged. You can't do anything about it, so might as well try to take care of it so it doesn't become damaged.
+ eat healthy - this is probably one of the best things you can do for your hair. You can read up online about foods that promote healthy hair.
+ stop using heat products on your hair and/or dying it. I know, it's hard, but it's honestly a lot better for your hair. If you have to use heat products, use them sparingly, so only occasionally like if you are going to a special event. Bleaching/dying, regardless of what anyone says, is not good for your hair (admittedly, I don't know much about henna dye, but your conventional hair dye/bleach products at your store are terrible).
+ Watch what products you use in your hair. This is such a rabbit hole; there is a lot of information online that says don't use this, don't use that, so you'll have to do your own reading. For me, I try to use products that avoid SLS, alcohols, parabens, parafins, preservatives and silicones (anything that ends with -cone or even polyquaternium).*
+ hair masks! Make your own DIY or you can purchase your own. There are recipes online.
+ Don't wash your hair often. I started off washing it every other day, now I wash it every three days. I know people who wash it once a week, it is something that you probably will have to work up to. Shampoo is essentially a detergent and strips your hairs of oils everytime you wash it. Your hair will produce more oil to overcompensate which makes you want to wash it even more. If you can "train" your hair to go longer without washing it, your hair will benefit.
+ don't wear your hair too tightly in a ponytail/bun if you have longer hair. It puts strain on your strands.
+ get your hair cut regularly. This was difficult for me to understand at the beginning since I had hair that grew slowly, but trimming the ends gets rid of split ends which can travel up the hair shaft and damage it. If you get rid of them as soon as possible, it can help hair grow. You can read online about "search and destroy," which is essentially looking for split ends yourself and cutting them from home, but I don't personally do this.
When you start to let your hair grow out again after damaging it for years, it will probably go through a phase where it seems like a nightmare, lol. My hair curled up so much after about half a year after stopping dying and starting to care for it. I essentially had a crown of curls that slowly grew out. I'd guess it took about two years to grow out the majority of my old, damaged hair - my hair did not grow that quickly then, now it grows quicker. This isn't something that happens over night.
Honestly, the absolute best things are just to eat healthy and stop dying/bleaching/using heat products. Everything else listed above is an "extra." Also, not everything I wrote about will work for everyone. Each person has different hair, different needs. It's a journey to grow out your hair and get it healthy again which means you have to try and fail and see what works for you. There's a lot of stuff on the internet which can be confusing (for example, the "controvery" behind silicones).
I've also tried hair supplements and didn't find that they worked for me. One of my friends who works for a compay that sells supplements says they don't actually work, unless you're not getting enough minerals, but if you are eating healthy (eating your vegetables, fruits, proteins, fats, etc...), then you should be getting enough through your food.
I'm not a hair-guru, but just try to learn as much as possible by reading about hair care. If you have questions, feel free to message me, or if you have any tips/corrections, I also appreciate those as well.
Have fun and remember, hair grows back.