For context, I stopped using shampoo when I was 17 because I started buzzing my hair (absolutely hated it, it was extremely unhealthy + I started experiencing male pattern baldness (which btw has nothing to do with shampoo, get that image out of your head!))
Back then, I had horrible dandruff, frizzy & flat hair, split ends, & it was extremely oily after only hours of not washing.
Now, I no longer have dandruff (of course still sebum & dead skin, but very mild), my hair doesn't get oily like it used to, and it can stand up on its own; Overall its just much healthier.
Routine before: Shampoo every 3-4 days (shower every day, just using water on the non shampoo days)
Routine after: Let water run through my hair, and gently massage it (essentially like shampoo, just without shampoo)
If you're just starting, expect the next 1-2 years to be disgusting, there was this one time where I combed through my hair not long after stopping shampoo, and there was a TON of gunk buildup, best just to buzz if you're a male. Eventually your body stops producing so much sebum and whatever, just trust the process.
BTW I'm not against shampoo, this is just my personal experience. I didn't have good hair guidance growing up which unfortunately lead me to abuse it quite badly, had I never got super dependent on shampoo, I think my hair would've been fine prior to stopping shampoo.