What can I eat to make my stool harder? I'm sick of having loose stool!

Add fiber. The first thing would be to increase the insoluble fiber in your diet, like bran or psyllium. Review your eating habits: the foods most commonly associated with loose stools are milk/ ice cream, fruits, salads/leafy greens when eaten to excess. Also moderate intake of caffeine and artificial sweeteners. Still having loose stools? See your doctor.
See your doctor. If you have chronically loose stools you could have a chronic bowel problem (inflammatory bowel disease, celiac disease or irritable bowel syndrome) which may be treated. You should see your doctor for further evaluation. Generally you should avoid dairy, salads, artifical sweetners and fruits. Starches (pasta, potatoes, rice) are helpful. Occasionally a fiber suppliment may be helpful as well.