。
You should start studying ahead.
Literal
Preparatory-study [object-を] begin-[past-た] side [subject-が] good be-[polite-ですよ].
予習 ('preparatory study, previewing upcoming material') is the complement of 復習 ('reviewing past material') — both are core study vocabulary in Japan, where the habit of previewing is culturally emphasized. ~た方がいい is the standard Japanese advice-giver: 'it's better to [already] have done X,' with the past tense 始めた adding a nuance of 'get it started.' Casual variants drop the です.