。
She made up her mind to do volunteer work.
Literal
She [topic-は] volunteer activity [object-を] do [decision-決心] [object-を] did.
The double-を construction here looks redundant in English — '~する決心をする' literally 'do the decision of doing X' — but it's idiomatic in Japanese. 決心をする ('make a resolution') is a light-verb noun construction; the inner verb (ボランティア活動をする) supplies the decision's content as a noun-modifying clause. ボランティア is itself a katakana loanword from English 'volunteer,' a postwar borrowing — Japan didn't have a unified concept of organized civic volunteering until the term was adopted, though spontaneous mutual aid (互助) has deep cultural roots.