It needs a cat litter box where it can do what it wants to do. You have to clean and maintain that box daily so that your cat do not go to any other place for excretion except its own litter box. Your cat will require some training if it is not used to use litter box but first of all give it a try by providing it a nice litter box. Also take it to vet if you still do not get it straight. May be it is having some medical problem and needs a check up. In that case you can give it Prozac to solve this problem. This is very common solution to this problem.

 It could have a medical problem, a pet will act out to tell you it don't feel good. Also you could try putting more than one litter box out for it, i have 4 cats and i have 4 boxes and they seem to prefer using one box to pee in and another to poo in.