What things can they lock me up in a nut house for?

"Nut house"? Psychiatric hospitals, or inpatient psychiatric units in general hospitals, are intended to help people who are acutely ill. Mental illness along with active plans to harm self or others, psychosis, severe depression with inability to look after self, mania in bipolar disorder, etc are all possible situations that might need inpatient treatment. There are longer term units for more severe illness.