smaq,
Patients who have pica are often nutritionally deficient in something.
You should first see your physician for an evaluation. It may be that you are missing a specific nutrient in you diet or you are unable to absorb that nutrient. Replacing it or bringing it into an adequate amount may cure your pica.
If it is found that you are okay nutritionally, then you will probably need professional help to manage the pica problem. Contact a mental health professional for advice.
Good luck.