Capital letters are used at the beginning of sentences.

Literal

Sentence [possessive-の] beginning [location-に] [topic-は] capital-letters [subject-が] are-used.

用いられる is the passive of 用いる (to use) — a formal synonym of 使う. The passive depersonalizes the rule — 'capital letters are used,' rather than naming who uses them, typical of written prescriptive language.