。
She's getting ready to start college.
Literal
She [topic-は] college [to-へ] entrance-do preparation [object-を] is-doing.
入学する takes the school it's entering with へ or に; 大学へ入学する is standard 'enter / start college.' 準備をする ('to prepare') is one of the productive [Sino-Japanese noun]+をする collocations. The full structure is 大学へ入学する (a verb phrase) + 準備 (a noun) — i.e., the relative-clause-as-modifier of 準備, 'preparation for entering college.' This kind of clause-modifying-noun structure is dense but very common.