Everyone is new at first, but they soon begin to think more highly of themselves than they ought to.
この場合、次のような言い方ができますよ。
ーEveryone is new at first, but they soon begin to think more highly of themselves than they ought to.
「誰だって最初は新人なのに、みんなすぐ偉そうになる」
to think highly of oneself で「自分を過大評価する」
ご参考まで！
カナダ 
アメリカ合衆国 
