最近ダイエットを始めた。

most recent; these days; right now; recently; nowadays

diet; losing weight by any method (e.g. exercise)

indicates direct object of action; indicates subject of causative expression; indicates an area traversed; indicates time (period) over which action takes place; indicates point of departure or separation of action; indicates object of desire, like, hate, etc.

to start; to begin; to commence; to initiate; to originate; to open (e.g. a store); to start up; to establish (business. etc.); to start ...; to begin to ...