Read the school's policy regarding music education. This is usually contained in the academic handbook, or guide of a similar sort. If not, you can ask your guidance counselor or music teacher.
Obtain a class list and look at the classes offered as fulfillments of the music requirements. Mark which ones interest you. Keep in mind your schedule as you do this, since not all classes will fit.
Think about instrumental or vocal performance, if this is a requirement or something in which you are interested. Surf the Internet and look at and listen to the various instruments available to you. If you can see and hear the instruments in person, this is even better.
Submit your choices to the scheduling department and your music teacher. Talk to him about your instrument choice. He may advise you on buying one or taking lessons if you want to get ahead.
Attend your classes, do your homework and practice constantly. Follow your music teachers' advice and do your best in class. Take lessons if you want the extra practice. Enjoy your concerts and take your tests. If you apply yourself, you'll easily meet the school's music requirements.