You will probably be asked first about what prior experience you have - particularly what prior custodial experience you have. The interviewer will probably also ask some general questions about whether you'll be on time, be trustworthy and responsible, etc.
Be prepared to answer basic questions about prior experience and skill. Also, questions about about why you are changing jobs. They will be interested in finding out if you had problems with your former employer since custodial positions often involve working alone after hours. Also they will ask about your schedule to see if you can work the night shift.
For a custodial postion interview you will be asked typical questions like to explain the experience you have as a custodian, why do you feel that you should be hired for this position, what do you feel are the duties of a custodian, and how do you feel you will perform as a custodian that will set you apart?
Being a custodian is a physical job that requires attention to detail, and I would expect to answer some questions about your physical ability to complete the work, as well as provide details on what you feel constitutes a properly cleaned area.