surprizo

The verb surprizi is transitive and it runs the way the event does the surprising: la novaĵo surprizis min. If you are the one taken aback you need the participle, mi estis surprizita, or you drop the root altogether and use mi miris. Writing mi surprizis means you surprised somebody else, which is rarely what a learner intends.

Surprize as an adverb covers surprisingly and unexpectedly, and surpriza is the adjective.