I have constipation and i'm dehydrated and eat 1 fruit a day could that be the cause and should it eat and drink better and if no improvement see a doctor?

Sounds like a plan: Each person with constipation needs to be sure there is no other disease causing it. If constipation is just due to slow movement of the food and poop through the gut, then the things to try are vegetables, water, fiber-with-water, prune juice, prunes, polyethylene glycol, Senokot or other senna, milk of magnesia, other laxatives... A doctor can evaluate and consider meds like Linzess.