Ask your school for a list of the books and details about the size/design/make of the table. Hopefully they will give you a couple of table options to choose from. Don't worry about buying sheets or towels before the first day of class. Part of your first day will be orientation and discussion of tables, supplies, etc. Anything needed immediately should be provided by the school.
While in school, Twin sheets will work reasonably well for most massage tables, though they will be a bit loose. You may choose to invest in fitted sheets later, but don't worry about it yet. Same goes for the mattress pad. Have fun in school!