How do I regather the strong ability of thirst

I usually drank coffee or no sugar soda when I was thirsty, to the point where I don’t remember to drink anything until my head feels like it’s about to fall of my body and I can’t barely stand up.

A science teacher told me it’s normal if you don’t usually drink water your body gets use to it slowly, but it’s bad for you.

Wondering how anyone with experience solved this. I can’t get a straight or incredibly accurate answer anywhere.