。
You should prepare for tomorrow's class.
Literal
Tomorrow [possessive-の] class [possessive-の] preparation [object-を] do should.
~べきだ is a strong deontic auxiliary — 'should/ought to' with a sense of obligation or moral judgment. Stronger than ~たほうがいい ('it would be better to') but weaker than ~なければならない ('must'). More commonly seen in writing or formal speech than casual conversation. Stacked の particles build the noun phrase: 明日の授業の準備 = tomorrow's class's preparation.