Of course I thought she would participate in the speech contest.
Literal
Of-course she [topic-は] speech-contest [to-に] participate [quotative-と] thought.
勿論 (もちろん) is the kanji spelling of 'of course' — the kana もちろん is more common in everyday writing. 弁論大会 ('speech/debate contest') is a Sino-Japanese compound. 参加する takes に for the event joined.