。
You have to study efficiently.
Literal
Study [topic-は] efficient [adverbial-に] do [must-なくてはいけない].
効率的 ('efficient') is a な-adjective/noun; its adverbial form is 効率的に ('efficiently'). ~なくてはいけない is one of several 'must' patterns — literally 'if one doesn't X, it won't do.' Contracts to ~なくちゃいけない or ~なきゃいけない in speech. する works as a light verb taking the adverb.