This depends if you are taking the Full IB Diploma or Certificate. Certificate students are only partially enrolled in the IB program, they are only taking a few courses so they can still take other Non-IB courses, while Diploma students only have IB courses in their schedule. This means that full IB Diploma students cannot take other non-IB courses.
You need to do this during course selection, where you choose your IB courses. If you are unsure on how to get the full diploma or just a certificate you can ask the guidance counselors present for help and they will explain it to you.